I see, sorry to tell you that there's no guarantee that MS won't change the 2501 number at some point in the future with another programming language update.
However, I think that's going to be the least of your worries in the eventuality of such an occurrence.
Those numbers have been fairly well set (as far as I know) for quite a long time, so there really is no reason to change them. What IS changing with each new language update (i.e. VB6 to VB.NET) is that the method of handling errors gets more sophisticated and powerful. IF Access eventually supports the .NET framework (which A2007 might do, I've been avoiding it like the plague so far, so am not sure) you will need to re-write your error handling code anyway, assuming you upgrade to A2007, so the 'magic number' will be, as I said, the least of your worries.
Regards,
Scott