SMBR will not stop functioning or be constrained in any way when your license count is exceeded. You should of course make every effort to get into compliance as soon as possible. Remember though, you don't actually have to buy SMBR to the same number of users. You can document a process (and it is entirely an honour based system) whereby only the 1500 users of your choosing will be permitted to have items restored. However, for the sake of an extra hundred I would suggest putting a purchase order in as soon as budget permits.
