I discovered an easy hack which lets you keep 8.1 without the upgrade. You do not need compatibility mode you only need to trick the client identity. Download a hex editor like HxD, open the msnmsgr.exe inside the Hex Editor. Search for the ASCII Text winnt (just the first instance, you do not need to replace all) Now replace the string of text from winnt to anything 5 characters long and there you go, save and exit. Messenger will now work with 8.1 with plus and all the bells and whistles without forced upgrade.
I hope this helps