No problem, an Early Upgrade just means you add $250 to the full subsidy price. You can still follow the Upgrade Swap steps.
There will be thousands doing the same thing next Friday. Are you planning to go to the AT&T Corporate Store to complete the Swap?
It should be simple to complete the steps at home by calling AT&T Customer Care but some reps don't always get the steps correct and one of the most common errors is the fact the the donor phone's SIM gets deactivated and will need a new Mini-SIM ( not a Micro-SIM ).
On your line there are no changes needed, no contract or upgrade eligible dates change based on the Upgrade Swap. Yes, you will be able to reuse your existing Micro-SIM if you have an iPhone 4 ( you could also cut down your Mini-SIM using the Micro-SIM as a template if you don't already have a Micro-SIM ).
With AT&T or other GSM carriers, the phone number is attached to the active SIM, so if you reuse your active SIM in the new iPhone 4S it will have your phone number.
Dave