I am asked this question a lot by people leaving one job (that is not a big firm) and joining a big firm.
My advice is that the year and the title (e.g., senior associate, mid-level associate, counsel, etc.) is not as important as the compensation. At my old firm (and I suspect this holds true at most big firms), my sense is that the top performer with three years of credited service was earning just as much if not more than the mediocre performer with four years of credited service. So, assuming your pay level is consistent with associates who share your law school graduation year, I would not sweat too much about the years of service for which you are credited.
But what about shortening the time to make partner? My sense is that the firm will make you partner when the other partners decide you are ready to be partner. Having an additional year of credited service next to your name does not guarantee a shorter partnership track.
My own experience is that I considered asking the firm to move me ahead one year because I have a CPA license and a Masters Degree. The partner who I worked under gave me the advice above — Namely, I was earning compensation consistent with a top performer in my class so there was no assurance that my compensation would increase or that my partner track would shorten if moved up.