[gentoo-user] Gentoo 32bit->64bit: How?

Hi,

My box is a working and fully configured Gentoo system, which is
uptodate.

For the sake of being able to address more RAM and for more
calculation power (mainly for rendering purposes) I want to
migrate to 64bit.

I googled for some tutorial but found nothing appropiate (one post
asked for the downtime to be expected while migrating a server --
something which not applies to me...).

My questions are:
1) Is there a performance gain, when migrating to 64bit if the 
   target applications supports 64bit?
2) Is it possible - if( true ){  how(); } - to """simply"""
   "convert" a 32bit system to 64 bit.
   "Simply" in my case means: Simpler ways than starting right
   from the bare metal of a virgin harddisk and doing the same
   stuff I did for the current system again... ;)
3) Is there some tutorial, which show me the path to go?

Thank you very much in advance for any help!

Best regards,
mcc