Bottom line, as pointed out above, a person with former military service wishing to join the Royal Marines, will still have to complete selection and begin RM Commando Recruit Training, day one, week one on the new entry rate of pay.
Partial pay-seniority maybe awarded dependent on relevant experience, usually on completion of training. The RM branch manager calls the shots on this element.
Suffice to say, plenty of RM Other Ranks join as graduates - there's usually two or more in each recruit troop.
With regard 'transfering' from a Commonwealth Armed Force, in all cases I've come across, the individual has to leave one to join the other as they are separate entities but the process may seem like a transfer because the person sometimes only joins one service a matter of days after leaving the other...if their current host unit permits them to complete RM selection whilst technically still serving in the overseas unit.