Even Topo 6 had to be updated to work with the PN-20, and since Topo 5 is older than 6, I see no reason for DeLorme to update it.
As far as I know, DeLorme isn't (yet) selling the PN-20 as a stand-alone unit, so your purchase most likely included the updated Topo 6 (labeled as "PN" on the DVDs). My understanding is that if for some reason you want to continue to use Topo 5, you can still have it & 6 installed side-by-side, but don't run them at the same time. They both want to access the files in DeLorme Docs, and don't share well.
If you uninstall Topo 5, all your saved work in C:\DeLorme Docs is not removed & can be used in Topo 6.