upgrading to amd64 cooker...

I just installed my Athlon 64 X2 and was wondering if I need to do a 
full reinstall of cooker to go 64 bit or can I mirror the x86_64 
repository and then use urpmi to do an upgrade. 

Also, is there any information out there on setting up the necessary 32 
bit stuff, like Firefox, to work?

Thanks
Scott