The gist of it is that reprocessing will top out at much less than 100%. For ore and ice, at a 50% efficient station, the max will be 72.4% with perfect skills and a 4% implant. To compensate for this, they are going to increase the refining yield of all ores by 38.1% (1/0.724). This keeps the value of mining essentially constant.
(Update: this is wrong. Thanks, Gev.)
Scrap reprocessing is going to be heavily nerfed. The maximum efficiency will drop from 100% to 55% (maybe a bit more with an implant; not clear). This means that a bunch of low-meta items will drop to about half their current price, since much of their demand is tied to mineral prices via reprocessing.
There will also be interesting effects on other markets. In particular, scrapping is a vital means for getting minerals out to null. This change will in effect abolish that practice overnight. As such, anything built in null will be much more laborious (more jump-freighter runs), or perhaps locally sourced.
There are two interesting changes for wspace (and null) dwellers. First, POS refining will be de-nerfed from far, far worse than a station (max yield of 75% and takes four hours), to be slightly better. An intensive refining array will get a base rate of 54% to reprocessing yield, as versus the 50% at a station. An outpost will be able to get up 60%! And it will take 10 seconds. (The current situation is insane.) So, good for CCP! I was planning to leave mining entirely, but now I am not so sure.
Second, the medium refinery will go away to be replaced with a compressor (acts like a Rorqual). I am not sure whether we'd use that or not; it depends on whether compressed ores are allowed into ore bays.