I expect that a large number of rooms will become available.
Think about what a "housing block" is. Conventions don't own the hotels. Rather, they enter into a contract with each hotel essentially saying that "if you charge $X per night, we will fill Y rooms for you." The sum of all the Y's is the housing block.
In 2021, Gen Con obviously struggled to hold up its end of the bargain. Downtown hotel rooms were basically available to anyone who wanted one, which had to be really tough on the hotels that were farther out and likely rented very few rooms.
For 2022, the housing block was likely negotiated during the Delta or Omicron waves, when COVID was raging. Nobody knew what August will look like, or what attendance would be. The hotels would be hesitant to block out a ton of rooms that might not get rented, and I know that conventions may suffer a financial penalty for failing to fill the rooms (although I do not know if this applies to Gen Con). Thus the size of the hotel block was probably very conservatively sized to avoid a repeat of 2021.
Now that the initial block has sold out, it should be relatively straightforward for Gen Con to go back to the hotels and ask them to release more rooms, and the hotels should be happy to oblige since the risk of many empty rooms has greatly diminished.