No, H6 is compatible with H5 projects. All the content packs of previous versions are still there, and work the same way as before. You can even download and use true legacy HALion content in the old formats (from the ftp site) if needed, and use that in H6. You simply get new content, better stability, and a long list of new features/abilities.
It should not be a problem unless you need to use a 32bit host, or run on a really old OS such as Windows XP. That should be the only reason you’d need to downgrade to older HALion versions. If this is you…then you’d simply apply for a legacy ‘add-on key’ for your dongle.
Example…
I can load all my old Cubase 7 and 8 projects (even if they were made with 32bit versions) that were made with HALion 4 and 5 in my existing Cubase 9/HALion 6/Windows 10x64 or Mac setups without issue. All the old content packs stay with me…all the old fxb plugin set-up information kept in older daw projects still work, etc.
Example…
I can still load old Sibelius 7.5 - 8.x projects that I made with HALion 5. Because the plugin id is the same…Sibelius doesn’t know the difference…H6 loads where H5 did before. It loads the project and parses my default HALion setup as it always did. I just double Check HALion to make sure it has the program table and other settings I want (I store these as multi-presets), and keep right on working as before.
Example…
All my old 64bit Bidule setups that I created with H5 still work with H6 as they did before.
Example…
I can NOT get HALion 6 to work with Finale 2012 and Sibelius 6 on my old 32bit Windows XP laptop. In this case, I simply apply for an H5 add-on key for my Steinberg dongle, and roll back to whatever 32bit version of HALion works best for my old XP Laptop.
So…unless you need to roll back to support a 32bit Host or and old OS etc…you just use H6…even with your ‘old projects’ that may have been made using previous versions of HALion.
Occasionally I fire up an old 32bit DAW or sequencer on my 64bit Windows 10 system. Rather than uninstall H6 and roll back, I just use a virtual MIDI loopback port, reroute audio through the DAW, and run HALion in stand alone mode (or better yet, host it in Bidule and use rewire and reastream).