The Church Missionary Department will assist you in changing your status to R-1 for the duration of your mission and then back again to F-1 status about six months before your release.
In order to return to BYU, you must arrange to do so with Admissions. more
To qualify to change from F-1 to R-1,
you must
be in good legal standing with USCIS (must have maintained full-time status while in school),
be a bona fide nonimmigrant and
have been enrolled as a student for 45 days immediately before the change of status.