Upgrading pets with Battle Stones.

[Notamonk]

Is it possible to upgrade certain pets like Peddlefeet or the Warbot? Like the ones that were already attained before pet battles were implemented and were automatically down graded to uncommon.

[Zaelo]

Yup

There's still several pets that only come as uncommon, I believe they are all upgradable with battle stones though.
Still looking for a mechanical one to upgrade my Mechanical Pandaren Dragonling, which is a good example of that.

[Stabya]

Yes - Used a Flawless Battlestone on Peddlefeet, and now he is rare, fast, and looks cool shooting his arrow.

He has a glass Jaw though, so you will be using that Stun alot