This seems to be happening alot lately. There are 8 (iirc) different service codes for Series2 DTivos, 9 if you count the R10. There seems to only be about 4 or so different 6.3e slices. It's been suggested that they're "consolidating code", so all that remains to be seen is which old service codes match to which new software slices. If people whose units have updated automatically would care to report, we can map service numbers to software slices to determine the slices to use for manual updates.
The 6.3e slices that I have seen reports of are 101,121,151,381, and 521. I think it's safe to say the R10 is probably the only one which must have the exact, correct, matching-numbers slice.
I've put together a table based on posts I've read here and somewhere else. Where it has been confirmed that a model self-upgraded to a non-matching slice, I've cited a reference. Until I see otherwise, I've assumed that models for which matching slices have been seen will upgrade to the matching slice, and I have listed them as such.
DirecTV Tivo Model --> Tivo Service Number --> 6.3e software slice.
Phillips DSR7000------------------> 101 --> 101, assumed
RCA DVR39-----------------------> 121 --> 121, assumed
Hughes HDVR2--------------------> 151 --> 151, assumed
Phillips DSR(704,708)--------------> 301 --> 101, confirmed, ddb @ showthread.php?t=54367&page=3
RCA DVR(40,80,120)--------------> 321 --> 121, confirmed, ddb @ showthread.php?t=56718
Hughes SD-DVR(40,80,120)--------> 351 -->151, unconfirmed
HR10-250-------------------------> 357 --> 357, unconfirmed
Samsung SIR-Sxxxx---------------->381--> 381, assumed
????????????---------------------->3F1--> ???
DirecTV R10----------------------->521-->521, assumed
????????????---------------------->5F1--> ???
The pattern I see emerging here is that the RID units and the non-RID units have had their code converged. Notice that (throwing the R10 out of consideration...call it the red-headed-stepchild) the RID units all have TSN prefixes starting with "3", and the non-RID units have TSN prefixes beginning with "1". Essentially, if the first digit of your TSN is a "3", change it to a "1" to get the correct software version, with the exception of the Samsung, which was not available in a non-RID "flavor. Perhaps a better way to explain it is to say that only the second two digits need to match (with exception for the red-headed-stepchild again).
I also noticed the second digit of the TSN prefix seems to correspond to the manufacturer, where 0=Phillips, 2=RCA, 5=Hughes, and 8=Samsung.