I went ahead and installed MR4 MP2 on both of my SEP Managers.
I added my laptop to a test group and set it up to download/install the updated install package from the SEPM. So far it has not upgraded.
I also noticed they added the URL field back in so that I can deploy using a URL to direct my client PCs too. So I went ahead and did that for a goup of 12 PCs. So far none of them have upgraded.
Possibly a flaw? Has anyone else tried this out with MP2 and experienced the same results?