me and a buddy tested it.
https://cdn.discordapp.com/attachments/389945345594884098/532343347420069898/unknown.png
If you use transformation abilities (elise, nid, jayce) casting the ult treats it like you actually transformed, letting you use all their abilities and ranges. It doesn't break either, you can stay in whatever form for as long as the ult allows you to hold onto ults (which is surprisingly long). When you recast the ult (such as to go in human form elise, hammer stance jayce, etc) it will convert you back to normal. No matter which form you ult (ulting elise while in spider) you can't turn into their base forms, only their ults (so no human elise or human nidalee for instance, only spider and cougar). Additionally you keep Sylas's passive, so everytime Elise or nid uses one of their abilities, you get that AoE auto on them rather than their traditional passives.
We tested jayce, elise, and nid.
Not sure about shyvana though, I know she swaps to dragon and uses her dragon form's abilities, but idk what the trigger for switching back is since its not an active to swap. Would like for somebody to confirm how LB interacts.
Bonus: Shaco's ult makes two clones of Sylas rather than 1 shaco 1 sylas. You can tell which is the real one because there's a little icon on the side of his health bar showing what ult he currently is holding, bummer but you can still mind game it sending out the clone like 2 minutes later when they don't know where the real you is and aren't expecting it.
Also karma's ult allows him to use any of her 3 abilities empowered once.