Hello Anjali,
Hope the Invoices are created & posted in your business at time of VF01. I mean if there is billing block at VF01 level & another person is releasing the Invoice to Accounts through T-Code VF02 then there is a chance that he can change the Invoice date.
Because in SAP Standard Invoice date can be changed in VF02, if it is not posted to Accounts.
Now, to the best of my knowledge, the User Exit - USEREXIT_NUMBER_RANGE is called at the time of createing Invoice (VF01) & not at time of changing Invoice (VF02), which means that authorized Users can still change the Invoice date before posting it to Accounts.
Thanks,
Jignesh Mehta