Bluejaw trigger .. male to female?
 

I know that many fish, including some triggers can change from male to female as the need arrises. Does it also work in reverse?

I have a mature Bluejaw trigger in my reef and I've considered getting him a mate. My LFS has an immature male for sale (it is small and it does have the yellow on the fins but only a very faint blue coloring in the jaw area.

If placed with a mature male, would the immature male change to female? Or doesn't it work that way.

It doesn't usually work that way, what usually is happening is the subordinant fish is just assuming a submissive pattern so it wont get picked on. There are studies of angels changing to female from male, but the fish is just returning to female color not actually changing sex. The only exception I know (and many may know different so please correct me if the info is different) is the egg trading fish like hamlets in which fish produce both sperm and eggs, but these fish do not show sexual differences. With that in mind I would not put a smaller male with yours, it will probably be picked on to the point of death, wait for a female.

From a human point of view, that sounds logical. I'll wait for a female.