That has nothing to do with whether the rifle is pre-ban, just the magazine itself.
If the rifle itself is pre-ban, you can convert to have a number of features a post-ban rifle cannot.
- “Assault weapon” means (a) a semiautomatic rifle that has an
ability to accept a detachable magazine and has at least two of the
following characteristics:
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the action of
the weapon;
(iii) a bayonet mount;
(iv) a flash suppressor or threaded barrel designed to accommodate a
flash suppressor;
(v) a grenade launcher; or