Out of these Southern non-Florida states, I think the order of flipping is pretty clear: NC>GA>TX
North Carolina obviously flips first for the reasons JRP1994 mentioned, then Georgia flips later of Biden is doing well enough, and then if Texas flips it means both NC and GA already flipped. That being said, the only one of these 3 I realistically see flipping in 2020 is North Carolina.
And on top of that, I could very well see Cooper carrying Biden and Cunningham in North Carolina this year. I think it may well be a reverse of WI/PA/FL 2016.
I can't see such an effect happening in GA or TX, and even if Biden somehow won those, the GOP would hold the Senate seats there regardless.
Texas seems to be a different situation entirely. I could just barely see something like this in a polls were wrong scenario (think 2000 where everyone was focused on the wrong states):
What do you mean by Texas is a completely different situation entirely? How so?