There is a special sale on right now for RM9 @ $20 US. See this forum post. RM9 is much improved over RM8 and is actively being developed and supported. Since you like RM8, the transition to RM9 is basically seamless and the current sale price is probably the best price you’ll ever see. Make lots of backups and you can work in parallel until you’re ready to switch permanently. There are several good posts on database cleanup considerations prior to doing your permanent upgrade. I keep falling back to this one. It focused on the move from 7-8 but is still applicable.
The only other thing to be aware of is that the 8-9 file updgrade will take your current RM8 file and convert it. So have some plan in place to avoid loosing your RM8 master. There are several approaches but copying all your RM8 data files into a new RM9 data directory structure probably makes the most sense if you are going to use both products for a while. That way you can play around with RM9 without affecting your RM8 data files. When you’re ready, copy your current rm8 master db file into your rm9 data directory and open the file in rm9. Whether or not you duplicate your media files is a matter of choice.