Hang on, am I missing something here? You're joining the Army next year and already you're thinking about leaving!
Do bear in mind that for 9 or 12 years your primary task will be that of a soldier (bang bang shoot shoot) and an aircraft technician second. That's the way the Army work. You'll need to do more than 5 years and do your 'upgraders' to come out as anything other than a 'tyre-kicker' so you really are looking at 9-12.
Also bear in mind that after 12 years you'll be coming out fairly high up the Army ranks (Sgt, maybe S/Sgt) and you'll suddenly be reduced to grubbing on the hangar floor again as you'll have no experience of any use to a civil operator. (Not much call for Apache maintainers on the North Sea).
Yes, you'll have basic experience under your belt but don't expect to come out and be feted as god's gift to aviation - because you won't be.
On a more positive note; the Armed Forces are working toward aligning their training packages with civil aviation to make it easier for people leaving to move into the civil side.
And as quichemech says - start studying early.
Good luck
Taff