I just re-read my post and I told you wrong. You DO NOT need another license to run user manager. You still get the packages from the download portion of mikrotiks site, the user manager package is in the zip. Sorry for not paying closer attention.
Thanks cbrown for the clarification - you are correct.
The other issue to consider before upgrading your license is NAND storage on the RB. Many need an additional microSD to successfully run UM, or Dude etc especially in a big site
You can check under the System Resource command.
If you run out of NAND space upgrading RoS, logging, backup etc will be a problem