MistPassiert in the teamwork video says that the lock-in happens when you join the war. That way you can set your mods as you want them for the war, join it, then move them for other things. (i.e. do it at your convenience, not having to leave them a special way at an arbitrary time). If you do major updates, you need to leave the war and rejoin it to get them reflected.
However, I do not know if that is correct. The way I would have read some of the other material is that the lock-in happens at the end of the preview phase. I would also be interested in knowing what really happens.
I did one test: during the preview phase, after joining I raised my gp. I then left the TW, and rejoined it. The active GP went up from what it had been before I left. I assume this reflected my changed GP. (Obviously I don't know what else might have been happening, somebody else might have updated something elsewhere which changed the GP, but that seems unlikely.)
Source: about 1:25 in to the top video in this message:
https://forums.galaxy-of-heroes.starwars.ea.com/discussion/144458/territory-wars-and-battles-schedule-of-events