Hi Uli,
Im not sure how to change the mask on the fly, but you can certainly supress the CPU compatibility checks by adding in a vCenter advanced setting ... I have used this method to save my bacon in the past - I will send you the exact parameters when I get to my desk, ETA 30min.
I have actually tested this by doing a live migration between clusters of different CPU family ... ie, live migration from Intel Host to AMD host and visa versa ... I wouldn't recommend this as normal operations, but there are times when this is really handy.
What it will give you is the opportunity to migrate a VM between hosts until you can schedule the downtime and reset the CPUID masks appropriately.
Cheers,
Jon
