This can be done with the console right now. Dismiss the follower in question from your service. They'll begin to walk away back to their "home" in the game, tavern, castle, whatever. When they do, open the console, click on them so their ID shows up ("000f42d3c4" or something like that), and then type
disable, return, and then
enable, return. Close the console and re-hire them and any companion you have should have their stats reset to the proper level to match with yours.
You shouldn't need to remove any items you have equipped them with but you can do so before you do this if you're worried about losing something important. Always save before trying this but it should work without any issues. You can check their stats before/after with the
getav command if you want to compare the numbers, ex:
getav health OR
getav twohanded and so on.