Ok, since I am a Cingular rep I will try to answer your question:
1) Cingular won't be FORCING anyone to migrate anytime soon...maybe in 1-2 years, but you still have plenty of time. Just take care of your equipment and your SIM card. You can always purchase a new AT&T SIM card for $25...or from eBay for a couple bucks
2) The migration process is very easy and quick. You are automatically given the new customer pricing on any phone, and there will be an $18 upgrade fee per line, just like existing Cingular customers pay.
3) Don't expect any "extra" perks from Cingular to get you to migrate. Cingular/SBC isn't in the business of giving away free minutes/extra cash off phones.