Could a Magent use False credentials to alter a coin from a lesser to greater value?
Sure, Light Glamour would be more appropriate, but it can affect 10 lb/lvl. That might be overkill. Though some may say that 1PP vs 2PP is next to no difference, it could matter.
There may not be many instances where money will enable "proper credentials for the current situation (which could include papers or documents)". But if you allow paper money in the campaign, I could see a
$1 bill to be falsified to a $20 (I think this was used in Firestarter to pay for a taxi).
If paper currency is okay, it could also be reasonable to allow it for a single metal coin; cover charge for entering a building. Other examples could be slipping the guard a silver to let you "see the prisoner" or a bribe that lets you out/into the city where all you have is a copper.
I would not allow a copper to be falsified to a mithril, or you could give a huge bonus to RR, as this would be less believable.
What do you think?