If Trump were to pick a different VP, I would assume it would not be public until closer to the election. If he is going to pick a different VP, Trump and team probably have it all gamed out as to timing.
I think Trump will appoint another candidate for VP. Even though Pence was a governor, his personality fits second-in-command much better. In four years the country will need another strong leader to Keep America Great. Absent a strong business leader of Trump’s ilk (good luck with that!), it will have to be a strong political leader. I don’t think there a military leader with enough popular appeal to slip in.
With honest elections, the 2024 candidate may not need incumbent advantage, but the experience of being an apprentice to the Donald will be very valuable.