Ground control centres access would only be granted to authorised employees, just like the cockpit access is only granted to pilots.
Make it all safe & secure & only authorised access allowed + to change aircraft GPS course would normally require cross checking & authorisation from more that one party or employee & possibly by 2 independant ground control centres.
Just like when you take out a large sum of money at a bank you need authorisation & checking / cross checking by multiple back staff & possibly the bank manager.
Checks & balances.
Chock Chucker.
