If I purchase a second class Eurail pass, can I choose to upgrade it to First Class on any given train, including the Glacier Express and Bernini Express? Thanks much!
Dunno. Why don't you ask Eurail directly here.
If you have a 2nd class pass, just reserving a 1st class seat is not a solution, you still need a first class ticket.
Upgrades are not really part of the pass. The individual rail companies each have their own policies for upgrades, whether it is allowed and at what price. That is for an upgrade to 1st class if you already have a 2nd class ticket. You don't have a ticket, you have a pass from a different company (Eurail) which then pays the individual train companies to let you travel on their trains.
In short, there probably isn't an option agreed between Eurail and the "Rhaetian Railway" and "Matterhorn Gotthard Bahn" for this. You need to contact Eurail.
I am thinking that you plan to purchase a Eurail pass through this website? As such, you are thinking that this is the correct place to ask a question about the pass? I can see why you might think that but Rick Steves doesn't sell the passes, Eurail/Rail Europe does. You should really read up on passes as it is unlikely that it makes good financial sense. Please read all the information on this link for the best advice - https://www.seat61.com/Railpass-and-Eurail-pass-guide.htm
There are Switzerland-only passes and travel cards that might make more sense for you if your trip is limited to or heavily focused on Switzerland. Although I've never used it, quite a number of people here have found the Swiss half-fare card worked for them for fairly short stays in Switzerland.
In addition, the Seat61 website details a way to get very inexpensive  tickets on the Bernina Express. You'll need to scroll almost halfway down the webpage I've linked to for that information.
For travel in other countries, a rail pass doesn't often make financial sense; point-to-point tickets are usually cheaper. If you let us know your full itinerary, folks who are familiar with rail fares in the countries you're planning to visit can probably help.
What exactly is your proposed itinerary?
We have only used passes if they make financial sense, these days it is a wash between buying a pass for a certain number of days and buying the tickets as you go. If you want to travel by a rail pass and want to upgrade to First Class, you need to go ahead and buy a First Class pass.
I second the Seat61 recommendation on dirty-cheap German-issue Bernina Express tickets. It is tricky, and relatively cumbersome to buy, but you do save 80% over normal prices.
Someone here tried the seat61 method, and found out that the tickets have to be mailed to you from DB, because there are 2 border crossings. It is not as easy as it sounds.
PS it was jules m
Sam, is correct. I purchased tickets for the Bernina Express from DBahn with his help. Honestly, I think I was unlucky. I would purchase the tickets that way again because the savings is HUGE. However there is a U.S. based phone number for DBahn that also sells tickets. They couldn't really help the problem I had with the lost tickets in the mail, but they gave me helpful advice. They claimed they were a "different department". I found out that I could have purchased the tickets for the same price from them (and apparently they could also handle the seat reservation) and then they would have been able to reprint the tickets if there was an issue. I had purchased the tickets online about 5 months in advance which gave me plenty of time to resolve the problem. I was just starting the process of contesting the charge on my credit card with the intent to repurchase the tickets via phone, and the original tickets came. They had been sent back to Germany and then resent.
I had to google for a U.S. based DBahn phone number. I think it turned up on a tripadvisor forum. What my advice would be is to "tinker" with the purchase on the DBahn website to get times/routes, and then call the U.S. based number to make the actual purchase. The process is described fairly well on Seat61, and there is a link. I had made some slight alteration to my route and that's where Sam was of help.
The train ride was delightful and it was worth the hassle.
Buying an upgrade to first class to go with a second class pass generally is allowed, except not in Britain. However, the cost of upgrading is the same as paying the difference in ticket price between second and first class. That could easily be more than $50 on the Glacier Express, depending on distance, which goes a good way toward simply buying a 1st-class pass, if that's what you prefer.
I happened to travel in 1st class on the Glacier Express but wouldn't consider it especially necessary. Every seat in the car was full, so it did not feel any more spacious and relaxed than any other train ride. Both classes have "panoramic" windows.
Hopefully you are aware the the "normal" trains also travel the same rails as the Glacier Express.