@Adminnnnnn: I scratched my head over that requirement, too. But the trick is that you can also install MR4 MP2 to a machine that has
no SEP installed, so do this--
- Leave the MR3 package in the GPO indefinitely, or at least until you are sure you have no MR3 clients, even those offline. (Actually, that's a pretty good practice in general)
- Add the MR4 MP2 package to the GPO. But on the Upgrades tab, click Add and select MR3 as a Replace install. That will uninstall MR3, reboot, then install MR4 MP2, giving you a supported install.
Caution: If you use OU Groups in SEPM, uninstalling may trigger the duplicate clients bug, about which there are other threads here, and a README paragraph describing a workaround available for it that's new to MR4 MP2. If you don't use OU Groups, I don't think you'll have a problem.
Going forward, if there is an MR4 MP3 or MR4 MP4, you can make those
Upgrade installs over MR4 MP*, and
Replace installs over MR3.
I'm assuming you're going to use the same GPO for MR4 MP2. If you use a different GPO for whatever reason, you can also browse other GPOs from the Upgrade dialog box to pick the Packages that the new MR4 MP2 Package will Replace (or Upgrade).
If you do this, make sure EVERY future package you add to the GPO has an Upgrade or Replace setting for EVERY previous package in the GPO, or you will add bogus entries to the managed install section of the registry that are a nuisance to get rid of.