MArcomage

it doesn't upgrqade durable cards like eternal guardian

Make a screen shot if you still can.

Else: sure you didn't just receive the Guardian at the spot of the Amplifier after playing it?

i only have eternal guardian and zombie, the zombie upgraded but the eternal guardian doesn't...

Maybe because some other uncommon undead card was already upgraded into a rare? (so the the Ethernal guardian couldn't be upgraded).

i can't tell you right now... if it happends again i take a picture :P