Hi,
I went back and tested this (I have a 5.1 vCenter with update manager, and I connected to an old 5.0 vCenter with update manager). The 5.0 plugin installed on my client and I was able to manage the 5.0 update manager, and it did not break the 5.1 update manager when I connected back to my 5.1 environment.
I know that is sort of the reverse order of what you are trying to do, but you can have both plugins installed.
When I look at my Program Files (x86)\Vmware\Infrastructure\Virtual Infrastructure Client\Plugins folder, I see both version of the plugin.