I like the idea of transforming weapons, it will make these weapons available even if you have very bad luck to get them from item boxes.
At the same time, i think that the weapons similar to their legendary counterparts should be upgraded to legendaries, like Red Firearms(common) can be upgraded to Red Shredder Beam(Epic) or Rocket blaster(Epic) can be upgraded to Huge Blaster(Legendary).
I also think that the power requirement to upgrade the transformable weapons should be reduced depending on the rarity, to make them transform easier.
A weapon should transform after it reaches the power level of 10 and gets enough power to upgrade it once more(that's why the power requirement should be reduced if this idea gets implemented, making legendaries more available to both new players, and old timers as well, providing a way to get legendaries at any level to all players)
However, I don't think that the common weapons should be upgraded directly to legendaries. In fact, it's better to make it in 2 or 3 stages instead.
I'll create a new topic with this idea alone, pls vote