#Japt, 61 bytes
I was working on this earlier but gave up when I read further down the post and realised conversion had to work both ways. Thankfully obarakon convinced me to take it up again and helped me save 2 bytes as well.
Takes input as a string for both operations and returns a string for both as well.
V="OIREASGTBP"UÉ?¡VgXÃe"%v%v"_i1'NÃe"%V%V"_i1'UÃ:Ur"N|U" £VaX
##Try it