Athens airport to Piraeus

You have options. The X96 Airport Express Bus to Piraeus leaves the airport every 30 minutes or so. There are taxis available. You can take the Blue Line Airport Metro to the Monastiraki Metro Station in Central Athens to connect with the Green Line Train to Piraeus. Since you have some time to kill I suggest the Metro option. There's a left luggage locker service just across from the main entrance to this station where you can stash your luggage while you have a quick look around before continuing on to Piraeus for your ferry.

thanks for the reply. I feel fairly comfortable with metro but
since we will have had overnight flight leaving us a little foggy,
would taking the bus be a safer option?

I definitely know what you mean about the long-distance fog! Since you would be going directly to Piraeus, I suppose it would be "safer" to take the bus, although I'm not sure what would otherwise be "unsafe". But what will you do once you're there? You'll still have to find somewhere to pass the time, but although there are some cafes and restaurants near the ferries, the Piraeus port area isn't the most pleasant.

Depending whether our flight is on time or not, I'm thinking might not leave airport
area until 4:00PM or later. However, if flight is on time, what are chances that the ferry line
would allow us to use ticket for an earlier ferry?

The ticket offices are right there where the bus drops you off. Just go in and ask.

In late October, we had Hydrofoil tickets to Hydra from Piraeus, after arriving from Chania on the overnight ferry. I deliberately booked them later because of concerns over the ferry being late and ignorant of the "fall back" time change that Saturday night.

We hung out for breakfast at a little cafe across the street from the Hydra ferry area and still got there before the earlier ferry was to leave. I checked with the office and they happily exchanged the tickets for ones on the earlier Hydrofoil because they had plenty of room.

I'd think you could exchange them if they have room which is more likely off-season like we were.

Thank you all