I think you got confused between a few different things. Pre-Kodi v19 on OSMC there was full hardware decoding for some formats built-in and additionally an option to purchase from RPi foundation keys to unlock mpeg-2 and/or VC-1 hardware decoding. With our Kodi v19 update we are no longer using the same decoding setup as the RPi foundation and others have decided they are moving to something new. As such there is currently not the same level of hardware decoding support and the unlock keys for the two formats mentioned above no longer do anything. So in the short term this may be a partial downgrade, but in time things should improve.
As for the HEVC option this was a mistaken setting that got set during an update that turned off hardware decoding for everything but HEVC and I also believe they reverted that change back to what it was since.
If you are unsure why not just image your SD card and give the upgrade a go. If you don’t like it then just restore the image back to what it was.