I got a signature card Grand Magister Rommath. And i already had him in diamond version. There was no reroll option where they said there should be… I am really upset that i can’t disenchant it!
No reroll makes sense as diamond cards are unique and have no dust value but it’s pretty poor form to make any card available as both.
You can reroll your existing normal/golden copy for free. Diamond cards are not affected by this feature.
More info about signature cards here.