So what I am wondering is, during the 24 hour preparation, I know if you change your war base, it changes on the war map, I understand that. The question is, when is the very last minute you could SWITCH war bases to a different one and it still count and use the new one in war? For example, on my maxed TH 5, I have a joke war base, everything spread so super super easy 3 star, and I have my base that I want to use, I like the people to think its a easy win, but last time I tried to switch it didn't switch in time so the free 3 star base was my war base for that war. So when is the last minutes you could change bases and have it count? Please answer fast, at war now.
I'm afraid there is no definite answer to your question.
At the very moment you switch the base on your phone, the information will take some time to be processed, sent to Supercell, and applied into the servers managing war bases. This delay can be extremely small, so in theory you could do it at the last seconds. But if a server is busy, some low priority requests can be queued for later processing in order to keep resources for important matters.
You will never know how long it will take for your change to be applied. So, I advise you to do it a few minutes ahead.