After purchasing 2 new servers we had problem to live migrate the virtual machines between the new and the old hosts but cold migrations worked just fine.

See error msg below

ErrorMsg

Server 1-2  Dell PowerEdge 2900 (CPU E5430)
Server  3-4 Dell PowerEdge R710 (CPU E5540)

Cause : CPU Mismatch

Solution : Override Masks please read this KB from VMware to fix this problem.

In vCenter Server, you can modify the default bit mask for individual virtual machines using the vSphere Client.
Power off the virtual machine before modifying its bit mask.To modify the default bit mask for a virtual machine

1) Launch vSphere Client and connect to the vCenter Server as an administrator.

2) Select the virtual machine that you want to migrate in the inventory.

3) Click Edit Settings on the Summary tab. The Properties window for the virtual machine appears.

4) Click the Options tab in the Properties window.

5) Select the CPUID Mask option to display the CPU Identification Mask information and settings.

6) Click Advanced to display several settings-related boxes, including CPU Identification Mask, in the right-hand pane.

Should be like this

Mask