UA sometimes opens NC (biz upgrade) space closer to departure date, depending on the load factor. Sometimes Upgrade For Cash is also offered at check-in.
If you are basing the 75% open seating using the seat map, keep in mind that it's not very accurate. Also keep in mind that a lot of C and D tickets are sold much closer to the departure date. Keep checking the C, D, and Z fare buckets to get a better indication of the load factor in the biz cabin.