I find that I can use grade 1 champions to upgrade my grade 3 to new ranks:

Nut when trying to upgrade my Nice grade 3 champ to rank 6 and wanting to use grade 2 chhampions at same rank it doesnt work?1

I have 3 grade 1 rank 5 and 2 grade 2 rank 5 but I cannot use the later ones. I would guess its a bug that I cannot use any grade as long as they are at same rank as needed.