After scouring Sl Universe and the official SL forums, it appears that some sims on the mainland are seeing a 50% prim increase. This is part of the big announcement that was supposed to be announced on Thursday, but as residents started seeing prim increases with the rolling restarts, they took to social media.
It has now been semi confirmed by Patch Linden: https://community.secondlife.com/t5/Second-Life-Server/Deploy-plans-for-the-week-of-2016-10-31/m-p/3077745#M17774, that the prim increases were no fluke and are permanent.
According to SL Universe, residents are seeing the following increases:
Estate Regions: 22500 prims up from 15000 prims
Homesteads: 5000 prims, up from 3750 prims
Keep in mind that these prim increases are on some mainland regions, so your sim may not have been upgraded yet. All I can say is Christmas has come early for some of us in Second Life!