I did just the opposite put a #1b on a #1s years ago.
The only thing you will have to do is open the barrel chanel up to fit the bigger barrel.Also when you put the forearm screw back in put a rubber O ring on the screw first then snug it up tight,this has helped with most of my #1's that i have had,cut some groups down by half.
I have had 12 #1s over the years,it's still my favorite single shot.
Frank S.
have fun shooting