Very variable, it depends on the type of unit e.g. Inf, RE, RA etc, what their gapping is like, the impression you make and ultimately how Glasgow view it. I've seen troops get offered continuation in terms of staying on for another tour, renewed FTRS (for over five years and counting) and direct transfer; though the latter despite being offered took over a year to come to fruitation due to Glasgow being unable to make their minds up.
Also it will depend rather on your rank and how long you have been with the unit. If you are very junior and/or new to the unit and you want to go fulltime you're probably best off joining the regular cadre proper rather than transferring where you may well end up doing your recruit course etc all over again. However if you have a couple of stripes there is a fair chance you may be able to skip all that and go in a Pte etc, it's unusual to retain rank if you are a JNCO.