Yes, you can just slide your stitch back onto the left needle and then restitch it using the desired purl stitch. Just tuck your needle into the stitch/loop that is in the back of the knitted stitch, put that on your left needle and then gently pull the just knitted yarn out of that loop.
Okay, reading that I'm not sure if I make sense. I can see me doing it but having some difficulty explaining it exactly. It is not difficult I just make it sound that way. Someone else can probably explain the "how to" better than I.
OTN: my first attempt at any type of mitten - doing fingerless
Oh...and I'm heajames on Ravelry