I might add the following - make sure you get a recommend for Reserves Service in your last OJAR. I'm assuming you are a serving Royal Marine already. You will also need 3 screenshots of your last fitness test, JMES MEDCAT, for which you have to be M1, and your security clearance.
Once you submit a seamless transfer request, you should go over on the same increment level however you may not necessarily go over in the same SQ.
With regard medcat, it depends whether you are joining a RMR unit as a generic Captain or whether you are aiming for a specific FTRS billet. If it's the latter, depending on the demands of the role, allowances can be made where necessary.
If you're in any way considering being a reservist after you leave then use the "seamless transfer" process via Firefly. If you don't like it you can always give your months' notice. However ex-regulars trying to join have all sorts of fun waiting for their prior service paperwork to come through which can takes ages - even if you only left the day before.