I take it since you have an ATPL etc, then you are no longer in the service.
If so then I believe it could be a more difficult process to get your instructor rating back.
For other people who are still in the service before you leave for the airlines it is worth converting your CFS QFI rating to a civi one, since after you have left, if you want to convert it is more tricky.
Sorry cannot remember the exact rules.
Lasors07 has all the answers!