This sounds more like a question for Ubuntu communities
I'm not an Ubuntu expert, however most modern Guest OSes I've seen tend to go the happy way you described.
To be on the safe side, shut down your VM, create a snapshot, add the vCPU(s), power on and observe the results. If you're not happy with them, just revert the snap, it will revert the VM hardware configuration as well (don't forget to remove the snapshot in either case).
If the VM is a part of multi-tiered app and it might not be desired to revert it back alone, you could disconnect the network adapter during the first boot after CPU change.
Hope this helps.