It will be roll the dice for each flight to find out what you get. Pre-mod craft will leave you on Geo, post will be on Leo. Hopefully Delta isn't so lost that they start mods before a viable constellation is in place, but that might be hoping too much. In which case you hope for a pre-mod craft, and get nothing on a post-mod.

At least this isn't quite as bad as when AA swapped providers. You could buy a monthly pass on the old provider which my employer paid for, but not the new one, and the new one was a pay per segment which wasn't reimbusrable because I had the monthly plan. Didn't know until on the plane which you got. Now the frustration will be the same, expect the pre-mod to get work done, get a post-mod, and watch a movie or read a book. Made planning productivity really hard.