How to go about downgrading to 3gs from iP4?

[Mjmar]

Is this possible even though the iPhone 4 uses micro sim? Do I need to call AT&T?

 

[terraphantm]

Go to an AT&T store, and ask for a new sim-card for your phone number. If it's a corporate store, they'll usually give it to you for free. Then pop the new sim-card into the 3GS.

I've gone from iPhone 3GS to iPhone 4 using the above method, I'm sure the reverse will be just fine too.

 

[SeniorGato1]

or get a microsim to full size sim adapter. I've used one to go between my nexus one and iphone 4. cheap on ebay. get the black one (not blue).

 

[Mjmar]

would it make sense if i used an exacto knife to carve out a hole for my microsim to fit in my old sim card, and stuck it in my 3gs?

 

[terraphantm]

would it make sense if i used an exacto knife to carve out a hole for my microsim to fit in my old sim card, and stuck it in my 3gs?

It'll work, but you'll need a little bit of tape to hold the two pieces together. That little bit of tape sometimes makes the tray stick in the iPhone in my experience