My understanding is R is also what UA ultimately uses for mileage upgrades (they use a different booking class but I believe it is based off of R) and what I've seen happen in the past is

- they can open up for mileage upgrades and then 0 them out during the sweep time.

I'm not saying that happens all of the time, but many folks here will say they've seen that happen.

Getting upgrades in advance certainly does happen in even the beginning of the first sweep -- but not all the time, and based on route, For example, hardly ever expect a LAX-EWR to clear way in advance.

Flying on the CO side of things for many years, getting the upgrade when it opens for the tier level doesn't happen all that often,

As a prior plat on CO now 1K, if I get the upgrade between 48-24 I'm extremelly happy but its often the case to get upgraded at say minutes before the 24 hour checkin or more often on popular routes get on the standby list and hope it clears before getting to the airport.