You cannot revalidate a civil IR using a military IR. However, the renewal requirements can be based on the expiry date of your most recent military IR (FW or RW).
See LASORS Section E 1.5.
I understand that most employers will require you to hold a valid civil IR when seeking employment.
HEALTH WARNING!! If you let your military IR lapse by 7 years, then you will need to pass ALL the exams again!