We've seen profiles where the 17a band plan boundaries are very slightly different, but I don't think they shift around at all.
With the 8c profile, BT followed the well-known 997 band plan, truncated at 7.05MHz. Alongside the rollout of the 17a profile, they've chosen to change to the 998 band plan.
I don't think BT actually have scope to go changing these plans either - I think it is specified in the ANFP.
The main difference in going from 997 to 998, in the existing frequency part, is that the band from 3 - 3.75MHz switches from upstream to downstream, which will be a change usable for many users, probably up to 1km distant. There is additional benefit in the new frequecies too - where the 7 - 8.5MHz region is downstream in the 998 plan, but would have been upstream if they tuck to the 997 plan.
So there have certainly been benefits to the downstream, but I doubt if we'll see any more significantg shifts in the near future.