You have to leave, rejoin, if you manage to pass selection for the cadets and get in about five years of summer camps and gobing off on army forums you might get a shot at a fish and chip mob TA infantry unit
Only after your engagement ends. If you Join up at the end of Your engagement then your pension rights and often your sucstansive rank are transferred with you. One of my Airmen from the squadron mortar section who was desperate to see some action did transfer. He kept his rank but was greatly P**sed off when he was posted to Germany and a lot of us went out to the far east to Sarawak to keep the Indonesians out (Confrontasi).