Your JCAB, once earned (not so much 'converted', you'll find out why) will not have any of your previous ratings from your previous license anyway, so where is the problem?
You are reading something in this that doesn't exist
To apply, you must have a current, unrestricted jet type on your current license, fair enough, but please quote me where this is exclusive of other ratings, unrestricted or otherwise.
Again, and this is important, how will you account for hours logged on a type you have had removed from your current license.