They will probably do it again in some way or the other with the rest of POS modules and the POS themselves.
If they didn’t do it, there would be a lot of protest from previous POS owners about reimbursement. The build cost for POS modules in terms of material were higher than even the buffed reprocessing rate. They could have either paid players out in ISK, which for the entire chain of POS stuff could easily be in the 2 digit Trillions or tried to replace old modules with their equivalents in times of Citadels, which would have been near impossible (many modules have been merged, build costs now could be much higher than before tec.) to do in a good way.
Increasing the Reprocessing rate to reimburse the players was maybe the only way.
I guess they are not totally unhappy with the pressure it will create on PI prices, but that’s just a theory.
P.s. To answer your question, they have to introduce changes in some way. The increase of reprocessing rate was indeed announced at Fanfest, just without a set date. Everyone who wanted to invest long-term could have done it if they have the ISK. For the announcement itself: well, they can just put it on SiSi or announce it here or on reddit or whatever, someone will always be the first to notice it and buy at the right moment. There isn’t really a way around it, unless they didn’t announce changes anymore - which would have its charme too. Not sure.