I read where Koa is scheduled to meet with the Dolphin doctors in May to evaluate whether he has fully healed from his neck injury. While this speculation that Zach Brown will replace him may be valid, there is also the prospect that they may both be on the roster.
If Miami was eager to cut Koa Misi, they would have already done it.
I think they're just hanging on to Koa until his replacement is either signed/drafted.