Forte keeps plugins distinct by use of a unique identifier based on the VST ID and the plugin filename.  Therefore, Halion 5 and Halion 6 are technically different plugins to Forte.  The following technique was reported as a method to quickly migrate from Halion 5 to Halion 6 using Forte 4.  There is no guarantee that this method will work for other plugin migration situations and it will not persist if you do a plugin rescan (which resets the plugin cache files).

Thanks to Joerg Haberstroh for this tip:

Forte creates a file named plugins64.xml. This is the plugin cache that contains the inventory of discovered plugins. I just changed the default path from HALion 5 to HALion 6 in the plugins66.xml file, so only the "5" with the "6" exchanged. Now you just have to be sure that all files and folders are exactly there as under HALion 5. Forte then loads HALion 6 and accepts all sounds in the scenes. It is important before the first loading of Forte that the computer must be restarted again in order that the correct dll is activated.