It all depends on the definition of CPU. If one will read the MileagePlus section on the UA website, it categorizes the YBM upgrades under the general heading of Complimentary Premier Upgrades.
The YBM instant upgrades are capacity controlled. Not all M fares are eligible for these upgrades. One must be a Premier Member in order to receive a YB or M upgrade. It does not matter what fare bucket from which the upgrades come. They are not completely automatic. These are therefore "Complimentary " upgrades.
Regardless of how you want to call them, the main point is that after Feb.1, as has been pointed out, Platinum and below Premier members waitlisted on Y &B fares will no longer be in the first priority level for upgrades within the 5 day window.
Is United attempting here to coax Platinum members and below to purchase Y & B fares even further in advance?
I don't think so. I also don't think that the YCA thing is an issue. As to the date of purchase, it really doesn't matter to UA. The only real issue here is likely that certs will come ahead of Y/B UG's for <1K's. There is nothing in the announcement which suggests to me that UA will toss the Y/B's into the general pool of what is often thought of as CPU's, e.g., any elite on a published coach fare.
As to YCA, if UA wanted to bounce those folks, all it needs to do is make YCA fares ineligible either for UG's at all or for Y-UP.