Shouldn't make a difference
I know I'm pretty new to this place, but been reading the forum for about 3 years now, as a guest, any hows.
My dads Simplicity plan worked great with my original iPhone, edge etc since he had unlimited data plan, but just upgraded his Simplicity to get my iPhone 3GS lol, so not tested it yet with my 3G phone, but his not complained, maybe he hasn't noticed it, but I know o2 txt me about updating settings on my original iPhone, so make sure your settings are correct in Cellular Data
APN: idata.o2.co.uk
Username: vertigo
Password: password,
any changes to contracts can take upto 24 hrs etc reps usually say
hope this helps