You do not change the Grant Expense GL account in either the unposted or posted grant. Account numbers get assigned to the voucher during the Grant Posting process as the AP voucher is created. If you need to change the Grant expense account (or the Payable or Cash accounts), it can be changed in Item Entry before posting all the way through Accounts Payable. So finish posting the application, but do not run the AP Edit and Posting process until you have modified your voucher.
The account numbers used in this process are stored in the fund class records (and fund GL overrides) in the fund module. When you create a grant, the accounts associated with the expense and payment of the grant will come from the fund class record associated with the grant fund unless there are GL overrides for the fund. For more information on Fund Class Codes, see FIMS Help Online and search on Maintaining Fund Class Codes.
If the grant is already posted through to Open Items, it can be changed by logging in with the NPO login (If you don't have access to the NPO login, contact FIMS support). After logging in as NPO, select the History supertab, find the grant voucher, and select the Line Item tab. The GL information at the bottom should be accessible to change. When you modify the line items on an unpaid grant, the new cash account will be used when the grant is paid. Typically the expense and payable accounts are not touched in AP since it will still be necessary to create journal entries to modify the general ledger. The expense/payable entry has already affected the GL and modifying the line items will do nothing to change the GL.
If the check has been created and posted but it used a cash account in AP that is different than the check stock, you will need to void the check (only in FIMS, and do NOT create a debit memo), then correct the cash account on the grant voucher as described above. Re-process the check (and do not print). This will align the check to the bank account of the check stock.