The set of creatures that gains indestructible is determined as Resistance Reunited resolves. After that point, an equipped creature that gained indestructible won't lose indestructible if it becomes unequipped. Similarly, one that didn't gain indestructible won't gain it should it become equipped.

If the target creature is no longer a legal target as Resistance Reunited tries to resolve, it won't resolve and no creatures will gain indestructible.