I don't believe a re-build is necessary when you expand VM resources although VM might need to be shutdown when you make changes to the VM spec.
If you think there is a good chance of exceeding 10K, I would use 3495 spec for the AM(s) from the get go and smaller VM for the PSN. That way when you exceed 10K, you never need to change anything to the AM VM, just build two new VMs and split out the Monitoring persona. The PSN can also be easily replaced with bigger VM if needed and this should be a very simple process.