When will it be easy for the hotel to upgrade its room type?

Time is hard to say, there are many rooms left in the off-season and will not be upgraded;

Ordinary rooms are sold out first in the peak season, and you may not even be able to book a reservation;

Hotels generally "accommodate" room upgrades according to their guests' luck; Sometimes it is the mistake of the front desk to arrange the room or the temporary decision of the hotel guest not to check out, and you sell the room to others just for the upgrade; Sometimes the sales department oversubscribes, and as a result, the same type of rooms are oversubscribed and only upgraded;

Another common situation is that guests complain and upgrade their rooms as compensation;

The answer to your supplementary question is yes. This hotel should be overbooked in the peak season. Since we oversubscribe, we have to take risks. In case all the reserved guests arrive, they will only be upgraded to the later guests. If the front guest is upgraded first and the back guest is absent, the hotel front desk is likely to write a report.