You need to make sure you have good balance with a treeless, and don't brace with your stirrups. Treeless saddles can easily slide side-to-side if you put uneven pressure on the stirrups.
They can accommodate a wide variety of back shapes and sizes, but they aren't perfect for every horse. You will need a GOOD pad in addition to the treeless saddle. Skito is a good bet - they are expensive, though (~$200).
If possible, I would recommend you try a treeless before you buy one. Action Rider Tack has a demo program, and has LOTS of treeless saddles. Valley Vet also has a trial program, but I don't know if any of their treeless saddles qualify.
I'd also wait until you get your mentor, and speak to them about switching. There are lots of things to consider regarding saddle fit (both treed and treeless) and hopefully they can guide you in the right direction.