JMC - FYI I'm trying the same thing w/ an old card just to experiment, only I'm paying it off in full at the end of the month. I'll keep you posted if anything out of the ordinary happens.
Yes keep me posted! I have had ZERO luck getting my scores to change no matter what balances I carry... and using one old card dinged me a few points, and using another old card has no changes. I'm about to give up, I don't think my credit will move at all! However, I'm interested to see how you make out!