Do i need to upgrade from 3.90 m33-3 to 5.03 to play my rockband unplugged umd?

I'm uming that the Sony firmware (you are talking about Sony's 5.03, right?) won't let you play the game images you have saved. You might be able to get away with upgrading to a newer version of the M33 firmware. A quick Google shows me that 5.00 M33 is out.