Here's an approach that seems to work way better for me (and my users I guess).
I just add an install.ps1 to the last version of the old package I'm going to provide, with the following:
param($installPath, $toolsPath, $package, $project)
write-host This package ID is obsolete. Replacing with netfx-Reflector
uninstall-package netfx-System.Reflection.Reflect -ProjectName $project.Name
install-package netfx-Reflector -ProjectName $project.Name
So that uninstalls the deprecated package right after installing it, tells the user, and installs the new one. Done. No dependency to the new package, etc. Clean way to get them forward IMO.