There are non-hold to maturity, which are treasuries and MBS that are marked to market regularly. Then there are HTM assets which are also treasuries and MBS that they don't mark to market. Then there are loans that are by nature HTM unless the bank is unwound - which also happen to have the longest duration risk. My guess would be that they left behind $90 billion in loans written at 2.5% because it is a waste of capital to tie it to that income stream.

The way you compensate someone for the increase in rates since the contracts were written is to drop the price (price and yields inversely correlated). So the HTM assets would be handed off at a steep discount (if they were handed off at all) to compensate FC for the new rates. If kept on FC balance sheet, they will continue to gain/lose based on market rates. Alternatively, they can be sold immediately into the treasury/mbs market which just turns to cash and is put on the balance sheet (they would realize no loss, personally, from this sale because the FDIC already took the loss when re-sold to FC).