Originally Posted by NickB

Yes. I would have thought that many would be in this situation. It would be unusual for someone to have already purchased all their 25-26 flights (especially as the latter part of the year is not on sale already anyway ) but it would be pretty common for some to have already made several bookings in anticipation on contributing towards their status renewal. Hence why people like orbitmic or karfa have already commented on the lack of (sufficient) notice for the change.
There is a situation which is, however, potentially even worse: bookings made prior to Dec 30 for travel after April 1st on OW airlines other than BA, IB or AA : the FAQs make it clear that TPs for flights on BA, IB or AA will be converted so as to keep their contribution to status unchanged but they are conspicuously silent with respect to bookings on other OW airlines, which would suggest that such conversion will not occur for these and that the new method of calculation would apply to those bookings. If that were so, we would end up not just with no notice but with retroactive effect, since bookings made before the announcement would be negatively impacted. Let us hope that this will not be the case and that the FAQs will be amended in due course to cover all bookings on OW airlines prior to Dec 30 and not just bookings on BA, IB and AA.