Just a heads-up for people trying to do this, you can transmute onto purchased “back brace” type items as well, but the side of the transmute window that you drop the tool pack onto apparently matters. This seems to be a known, but low-priority, bug. I just recently successfully transmuted my pack onto a forest brace purchased on the TP, so I could finally have back stats and the upgrade slot, without losing my hobo pack.
So if it doesn’t work for you one way, try reversing which side the pack is on, and that may fix it.
They might have fixed this. I just tried it with a forest brace (lowest I could find on TP was lvl55 green/masterwork)… it soulbound on both the left and then on the right side of the transmute window.