I changed the 4Gb to 8Gb - had to pay the restocking fee they repeated like 50 times I'd have to pay the restocking fee - like they had to remind themselves - or thought I was creazy to actually accept to pay 50 bucks.
Had an argument with them - they say 10% restocking fee, but charged me $50 even, instead of $49.90.
They looked at me as if i was creazy - I told them i was already paying too huge a price for my impatience, to want to loose an additional 10 cents told them my bank charges my $35 if i go overdraft of 10 cents (also pointed that, if out of all these new customers they had at least 100 000 people who returned their phone, 10 cents would becomes $10000 which is not bad for a week end ....)
Fight for your 10 cents!!!!!! - but exchanging the 4 for the 8 is possible and you DO NOT need to change the SIM - Itunes activates the 8Gb, deactivates the 4Gb, syncs everything the same way your 4 was - and off you go - changing the phone took me 5 minutes ---- very easy
oh - and 4 Gb more MAKES a difference it is clear that with the iphone , you actually HAVE to manage your library - while walking around with 250Gb of music doensn't make much sense, movie + podcasts + a few songs DOES take a big amount of space and 4Gb os not enough for a trip, for example. 8Gb gives you enough room to have some entertainment ready while on a plane, etc.