The only difference between Dec and Jan onwards is the provider for the GMAT. Pearson Vue takes over in Jan 2006 from ETS. Currently, there is no plan to change the test format (that is, it will still be AWA, Quant consisting of PS,DS and Verbal consisting of SC, CR and RC).
However, Pearson will do away with the scratch paper system used by ETS. In its place will be white boards (which some members have commented is nothing more than a laminated sheet of grid paper) and fine markers. I'm not sure if it will affect your test taking strategy, but it sounds very cumbersome from what I have heard so far.
Still, your main focus right now is to get the basics right, make sure you know what you're doing and prep yourself well with some full lenght tests. Don't let the change fo system rush you into taking the test without being fully prepared.