I believe the OP's situation is this: Many Advanced programs have discovered that people really, really like a guaranteed prelim year -- it's more convenient, plus it's a huge plus for the couple's match (where the prelim match is not couple's matched to anything). Therefore, previously Advanced matches are working out deals with their home prelim program to offer a Categorical match -- 1 year of IM, followed by X years of the other specialty. Anesthesia fully embraced this model, but actually built a specific prelim experience (the CBY year) and offers both Categorical and Advanced. Neuro is now moving this way, especially because it's a buyer's market -- all the neuro programs are competing for the best applicants, and a Categorical match (instead of Advanced + take your chances in the prelim match) is seen as a big plus.
Assuming this is what happened (i.e. the OP matched into Categorical Neurology or something similar), the answer remains the same as above. As far as the NRMP is concerned, you matched only once -- into your categorical program. You have no match commitment to start your "neuro" training, and you don't need to worry about the Jan 15 deadline. On the other hand, as a professional it would be best to do the "right" thing. If you're going to give up your PGY-2 spot, it would be much better to do so before the match so that your "neuro" program can look at their applicants, see if someone could start a year early, and offer them the spot.