There are some pro's to this alleged change. But, this 10-hour cutoff is really unfortunate. I've had paper certs and at t-24, it was J0 for segment 2. When I arrived at connecting airport, seats opened up (presumably misconnects) and I was able to use the paper cert and board. Under this new system, this scenario would not work out.
Devils devils advocate though, the reverse would be advantageous to have a confirmed J seat booked weeks in advance, only to find it is booked 100% or over on day of departure - where the paper cert wouldn't have a chance of clearing on day of.
What did the market research the UA did (interviews with flyers, focus groups and such) have to say about this proposed change?