I know Kovalainen was replaced by Rossi in FP1 in Barcelona, so maybe it's nothing. But I remember seeing rumours that Petrov didn't have the support for next season so his seat was in doubt, so it'd make sense to me to replace Petrov unless Kovalainen was on the move.
Edit: Actually, more on that from Caterham themselves:
If he's got a cold, why not swap around for FP1 and give him longer to recover? Seems an odd choice, but nothing significant. However, Giedo getting more time in the car could be an indication that he's in the frame for next season, I don't remember them announcing this before, so to suddenly decide to give him 4 FP1 sessions (and no more to Rossi) might indicate he's in the frame for 2013.