Transportayion to Marco Polo airport

That time in the morning I think you are pretty limited to a water taxi. It's costly. Your hotel can arrange one for you. The Alilaguna (water bus) does not run early enough to get you there.

That's a significant flight change (more than a few hours). You have the right to call them and ask them to work out a better schedule for you (look online for the second best alternative, including times and flight numbers) that doesn't involve additional transportation costs on your part. I've had this happen many times and I immediately got the change reversed into something that works better for me. I got a "no" once and I just called again later and got a more helpful person. Be friendly, but assertive and have the alternative flight info ready to spell out to them. They will work with you if their change poses a burden on you, and it's more than a minor change (more than 2 hours).

Many travelers in Europe get departing tickets on early morning flights, however they're just going to be flown to another larger European gateway city where they sit until late morning. Most of the direct flights from Europe to North America are certainly not leaving at 6:00 a.m. to 8:00 a.m.
I agree about calling the airline and requesting a change--even if the have to send you to a different gateway city to fly home from.
When we flew out of Venice last April, we flew British Airlines 10:00 a.m. into London Gatwick and left from there. And the airport was slammed because we (and a few thousand others) arrived by cruise ship and were wanting out of town at the same time. We made an appointment for a taxi to take us from the ship docks to Marco Polo Airport. The vaporetto going to the airport was 15 Euros x 2, and took longer.

Unless you are locked in to your Venice overnight accommodation, you could book at a hotel closer to the hotel that would be served by taxi or car service. For example the Courtyard Venice Airport is located about a km from the VCE terminal. The Courtyard Venice has a shuttle service:

AIRPORT SHUTTLE BUS FROM AND TO MARCO POLO AIRPORT The shuttle service
fee is Euro 2.50 per person per ride Reservation for the shuttle bus
from Hotel to Airport has to be done at the Front Desk upon Arrival –
Shuttle available from 5.00 AM

You might do a little more searching on this site for answers. My only experience was in January two years ago - most likely less busy than April. However, the first Alilaguna does leave San Marco about 3:50 and gets to the airport stop about 5:09 (winter timetable). It's about a ten minute walk after that but not difficult. We were on a connecting KLM flight but what I read from posters here said the airport didn't really open till about 5:30 and that was what we experienced. Our flight left at 6:30 and there was plenty of time. It's not particularly fun to be up and waiting at the dock that early, but if you are flying somewhere else and connecting you ought to have time with a 7:40 flight. Still worth seeing if you can get a later departure, though.

It is not fun. Call the airline. If not, get request a water taxi

I think you'll find this post helpful as the departure time is the same: https://community.ricksteves.com/travel-forum/italy/getting-to-the-venice-airport-early-morning

Agnes is right. Call AA and tell them you already have an early morning tour in Venice booked and paid for and you will lose the money if you depart that early. I've had this happen several times as well but I called and resolved it to my advantage. One time we were supposed to fly out of Milan to Paris and then onto LAX with a 2-hour lay-over in Paris but the airlines decided to change our flights and have us fly the night before into Paris, then depart the next morning for LA which would have decreased our sightseeing time in Lake Como plus it meant we would have had to leave the airport in Paris, find a hotel and then make our way back to the airport the next morning (a lot of schlepping!). I called and successfully got our flights changed back to our original 2-hour layover in Paris. It's worth a try so you can maximize your time in Venice and it will give you more options on getting to the airport.

I am responding to all of your suggestions on changing flight times and as a notice to anyone traveling on American airlines to and from Venice. When I first did my flight reservations for this trip, American had a direct flight to and from Philadelphia. They no longer fly this route. The only way to fly American Airlines to and from Venice now is through Iberia or British Airways. They tried to change me to the early AM flights after the first schedule change but I resisted and got the British Airways afternoon flight. The British Airways fight connects in London then to JFK. The next schedule change was with my connecting flight from JFK to Raleigh, NC. They changed this flight from 9:00 PM to 8:30 PM which changed the connection time with the British Airways flight to 1 hour and 25 minutes which violates their requirement for a 1 hour 45 minute connection. Thus my only option was the earlier flight.

Trentriver, Egads, this is really fishy!
On Jan. 24, American sold an economy ticket on this flight to my son,

AMERICAN AIRLINES AA715 Depart VCE Venice May 15, 2015 Arrive 12:15 PM PHL Philadelphia May 15, 2017 03:55 PM

...and they still are offering the flight on their website on that date at a decent price (and on April 25 on the AA website but not cheaply).
One wonders if they are telling you the real reason for the change, and if you have any avenues of protest.

OP and Mike, please let us know what happens. We have the same flight from Venice-Philly-Raleigh in October and (knock on wood) haven't heard anything from AA about that changing. We used more frequent flyer points to book this one, as I knew my last night would be ruined if I was worrying about getting up at 3 AM so might have set up the trip differently if we couldn't book this later flight.

I am seeing different things on the AA web site. Venice - Philly direct has always been seasonal service and they run it in 2017 from May 6 - August 6. If you look at this map, click on "Flights" in the upper right area and enter VCE and PHL as the origin and destination. Take a look at each month and it shows whether there is direct, connecting or no service.

http://aa.fltmaps.com/en

However...If you want to buy a ticket for October 10, it shows the direct flight. So, I'm not sure what is going on!

An easy way to view an AA flight and set up a notification is here: https://www.aa.com/travelInformation/flights/schedule

Trentriver, I don't know if you'll still see this post, but what happened with your Venice return flight last month? Our return flight in October is the same one you booked, and knock on wood, it is still set for the time we purchased for 12:15. Thanks!