When you enter an application you will typically enter in the grantee id and perhaps the grant date and amount and then you enter in the fundid. When the fundid value is entered it automatically creates a single record on the payment plan tab for that fundid with a payment date the same as the grant date. You typically then save the record.
If you then go to the payment plan tab and add line items, the fundid on the main Application tab will now turn gray since there are multiple payment lines. If you need to change the fundid on the application at this point, it will be necessary to change the fundid values on the payment lines in order to affect a change on the front page of the application record.
Note that the front page of an unposted application's fundid field will display the fundid on the top line of the payment plan