I just moved a toadstool leather this weekend that was getting too big and was pressed up against the front glass when it was fully extended. It had attached itself to a large piece of base rock so the only way I could reposition it was to cut it.
I took a sharp knife and cut it off the rock. I removed it from the tank, then cut the "stem" off square. Then I attached it to a smaller piece of LR using the "toothpick method." I basically put two toothpicks through the base and then used rubber bands to hold it on the new rock. I've done this with a Sinularia sp. and it attached to the rock completely after a couple of weeks. I've never done this with a toadstool leather, but I don't see any reason why it won't work since they're both leather corals.
BTW, you probably know that toadstool leathers are picky and when they get hacked off, they will "wilt." My toadstool is wilted now, but it should be doing much better after a few days. It also is in a different part of the tank, so it's not only pissed off about being cut, it's also got to reacclimate to the new position in the tank.
Good luck and keep us posted.