You could change them anytime, but I would finish off what they have. Brahmas take their time when it comes to maturing! You probably have several more weeks until they lay and as late as it is they may wait until Spring.
Finish the bag. You can either leave them on the starter or move to a grower. You probably will not see eggs until 26-28 weeks with Brahmas. You should get decent production with them through the winter.