Matt has it about right.
To change from RM to RN (or t'other way around) is fine before you join, BUT you cannot change after you've joined- in most cases you have to leave & re-apply.
You'll need an Update Interview (shorter than the first) to ensure you are suited to the job which you now wish to apply for. Bear in mind, you could be suitable for your initial choice, but found unsuitable for another job regardless of which job you want to transfer to/from.
It's important to identify a job which actually suits you as an individual rather than just leap at a job that gives you an easier & quicker route to employment- there are many that opt for a quick fix & subsequently regret it big time. Please think about it very carefully & don't become one of the many that join the RN in a shortage category branch, then try to join the Corps because it won't happen if there's a manpower shortage in your trade.
To give you an rough idea: your selection interview is valid for 6 months, eye test, medical, Security Clearance & PJFT 12 months, Recruiting Test 3 years. Any element that expires needs to be re-done, together with your interview if you change your Branch of Preference.
