Mine is a clone, but I did the reverse and changed to a 12AY7 as per the 57 Custom. Gives me better control of the volume and if I want the extra crunch of the 12AX7 I can use a clean boost. I'm not sure if the particular 12AY7 I used has made it sound slightly smoother/a bit duller than the 12AX7 that was in it before (I have added the 25uF cap that is fitted to the Custom Champ, but missing on the clones, even though there is a place on the board for it).
It's a cheap enough mod to try and obviously can be reversed easily too. I love my little Champ, though I still find it's too loud for certain places to get it into proper crunch territory.