Like you I also have the $5/200 msg grandfathered plan.
When I placed my order on the Apple store (just a few minutes after it went live), it let me keep my $25/2GB data plan, but for texts the only options were pay per use or $20/unlimited.
Since most of my family and friends are on iMessage, I rarely use more than 20 or so texts per month, so if I lost the $5/200 plan it might actually save me a little money, but I'd prefer to keep it anyway. Let's hope it stays on or AT&T can add it back on after I activate the new phone. I definitely don't need unlimited texts or even voice, so the share everything plans would cost me significantly more.
I find it very odd to see the number of different experiences people had on the Apple store with AT&T... some were forced to upgrade or change their data plan while others were able to keep their old plans, some were offered to keep the $5/200 msg plan while others were forced to change, etc. It seems very inconsistent for an Apple experience.