Any impact with AT&T contract if I go from 3GS to first gen iPhone

You signed a two year contract for a subsidized 3GS...you can downgrade when your contract expires...more than likely even if you switch you'll pay the data price that's commensurate with the 3GS...what benefit does using a first gen iPhone have?

 

Check the contract

I'm not sure, read the agreement clauses to see what they require you to have. If they just have it written that you must comply with a data plan (w/out) price specifics you could in theory ask them to downgrade you to the cheaper iPhone edge unlimited plan w/ 200txts for the orig iPhones if you get a nice person...
Also remember that in the contract they state that they can do any change they deem necessary (for their favor) so they can simply say pay the $30 even having the old phone since your contract was done w/ a $30 data plan...

 

You shouldn't have a problem. You agreed to a two year commitment to AT&T, not your iPhone. If you want to downgrade, you shouldn't have a problem. Pop your 3GS sim into the iPhone sim slot and you are good to go. You will probably need to call AT&T to switch the data plan down to the iPhone data plan, but other than that, you should be good.

 

This is it. You can even switch the data plan yourself by getting a brand new sim card and restoring the original iPhone as new. Pop the new sim in the phone and connect to iTunes. It will ask you to activate the "new" phone. Go through the steps and when prompted, you are replacing your current phone with this iPhone. Just choose the $19.99 +200 SMS and then you are good to go.