How to swap MB+cpu between 2 systems - 1 athlon, 1 P4?

[rgbtxus]

Hi,
I have a gentoo system with an athlon 1.4MHz and a windoze 2k system with a P4 2.4MHz. I'd like to swap the MB+CPU+Memory between the systems. The gentoo box has CFLAGS="-march=athlon -O3 -pipe -fomit-frame-pointer". I cannot afford for either system to be down for more than about 1/2 day.

I assume if I just do the switch and boot a new kernel the system will probably not run (or run much) as some athlon only instructions will try to be executed. True, right?

I don't have a third box to build a new gentoo on (and besides I have tons of customizations I'd just as soon not have to redo). So, one idea is to set CFLAGS to "-march=i686 -O3 -pipe -fomit-frame-pointer" and do a emerge -e world. After 3 days I should have a system that will run on the P4. I am correct in assuming that minus the compile load the system will remain fully available?

Then, I can do the swap, reset the CFLAGS for the P4 and do it again. right?

Seems like a waste of energy. Is there a better approach?