If you're in the same major category and just changing sub-category, you don't have to let HQ know (Budget, FO, Other, etc). If you need to change major categories, MLS should automatically generate a message to send to HQ informing them of the change. Once you have done the switch, you can check your sent messages to make sure the change is being sent in. If not, you'll have to contact them.
Once the change has been made, HQ will send you a message back confirming the switch or notifying you of any complications or further steps you must take. Finally, it will also show up in the miscellaneous section of your CUFS.