Anthology of heroes with accessories
No, you can’t do it. If you don’t need them, just salvage them. You only can change stats of the new bloodstone ones.
If you’d be able to change the stats of ascended trinkets that way, it would completely nullify every other way to obtain ascended trinkets. It would make half of what the laurel, guild accomodation and fractal relics vendors sell redundant. ANet is not going to do something like that. I believe this is also the reason why we can’t craft ascended trinkets, since that would have the same result.