Ok, this is what happens if you give an unfocused individual like myself an idea: I have now implemented juliaup self update for the Windows Store version. Running that command is equivalent to manually going to the store, checking for updates and then installing an update for Julia if it is there. The version of juliaup that has this is 1.5.0, and it should show up in the store in a few hours.
No progress on the Linux and Mac side is the downside of this 