J,
This is a fun situation. The best document on EVC is VMware KB: Enhanced vMotion Compatibility (EVC) processor support The simple answer is yes you can avoid downtime as long as they have shared storage (both host can see the VMFS luns) and your setup your EVC correctly. You will want to setup EVC for the highest allowed setting for all your hosts. Then when you migrate off the old hosts raise the level. I would really try this on a test VM first to make sure you don't get any downtime from the action.
Thanks,