changing roles before recruitment test


I have applied for the role of Marine Engineer in my application and have now been booked in next month for my recruitment test, I was wondering whether it was possible for me to change the role I have applied for to Royal Marines commando before the actual test itself or if this is something that can be discussed & changed at the interview. I did start a separate application with a different email for the RM, but should I just withdraw that and wait till after taking the recruitment test? Thank you