Not sure what you mean by "overwrite the EFI". The only way to go back to a previous OS version is either via a Time Machine backup or a clean install. If your system is working with Yosemite, I would suggest backing up your files first, and go for a clean install.
I installed a clean yosemite, I upgrade to 10.10.2 but there are too problems.
So I try install Lion on new hard disk and it doesn't work.
I was afraid there had been an update to the System BIOS level and you could Turn back.
I try an extreme way.. I try to install Snow Leopard and it works.
Maybe my Lion was corrupt?
I put Mavericks on mine and I think it's a good compromise. Lion had several issues related to network sharing that were really impeding my systems role as a server. Particularly SMB sharing. And the screen sharing bug.
But Mavericks has been great. I use a very cheap ATI HD5450 video card ($30)
Just get Mavericks running on the Geforce card. Then install ATI card in another slot. Follow those directions and make sure OSX recognizes it. Then shut down and remove the Geforce and put the ATI in the bottom slot.