Nope. You're gonna end up with a mission where it pops up and says "There's no going back" and you can click okay or cancel. If you cancel, you can still switch sides.

Ahh okay, so I can still help the Brotherhood rebuild Liberty Prime and destroy the Institute and then betray them?

No. You have to confirm for the Brotherhood before you finish Liberty Prime.

You can build Liberty Prime. Just don't destroy the Institute using the Brotherhood. You want to do that with the Minutemen. The ending of the game if you're not the Institute = whatever faction you use to destroy the Institute

EDIT: Was wrong for crossed out section.

Okay, I understand now. So, how do I get the quest to destroy the Insitute with the Minutemen? Since I didn't give them the tape...

If you've done quests with the Railroad, one of the quests was to make a second tape, so the Railroad also has one. Anger the Institute however you like (e.g. shoot em' up ) , then talk to the Railroad. They'll tell you to rely on your own forces then and give you access to the tape.

Hmm okay, that might work. What about this? Could I destroy the Institute with the Brotherhood and then kill Maxson or something and get the quest where Garvey has you shoot down the Prydwen?

From what I heard, that should work.

All in all, you are never really locked into an ending, just out of it.

As Akul said, I don't see why that wouldn't work.

Just get 5 artillery pieces and go shoot up the airport but be careful.

At what point are you "locked out" of an ending? I am trying to figure out where to make a save point so I don't have to play the whole thing again to get all of the achievements. (I know, I'm lazy).

First point that you're forced to make a faction hostile (in other words, locked out of an ending) is a quest called "Mass Fusion". You can make factions hostile before then by doing some blatant things like kill a ton of their members at their headquarters in middle of a bunch of witnesses, but this is the first storyline mission the choices you make for that mission make someone hostile. A save where you first get Mass Fusion is a pretty good branching off point.

There is an "exception", however. If you really want Paladin Danse's perk but want to side with the Institute, complete Liberty Reprimed before completing the "Talk to Allie Filmore" objective of Mass Fusion. That's because Liberty Reprimed and the next objective in Mass Fusion require you to talk to the same NPC in the Brotherhood. If you talk to that NPC, they will not talk to you about Liberty Reprimed until after Mass Fusion is done, and you don't have a choice. Mass Fusion forces you to either make the BoS or the Institute your enemy.You need to do Liberty Reprimed for the perk because a quest that will be given after Liberty Reprimed is given and that quest's completion is necessary for the perk.

There's an easier way. Just go talk to Preston Garvey and he'll ask you to retrieve the tape from the BoS. The BoS will just give it to you and then you take it to Sturgis or whatever his name is.