T5 introduced several ways that PCs could switch.
Rogues could do a 'masquerade', allowing them to run an alternate identity in another career for up to 5 terms. Mustering out was done at each masquerade, and mustering out of Rogues only took into account terms served as a Rogue proper.
Agents could go 'undercover' every other term, under an assumed ID, in another career, for one term. Since it was an assignment, it didn't really count as another career: although you rolled the dice and picked the skills from that career type, you were still at the core acting as an Agent. Therefore, mustering out took all terms into account, and was only done at end-of-career.
Imagine your PC as a retired Agent. Not as a corporate ambassador, like some Agents, but an espionage expert. First he gains some skills, then he goes undercover as a Marine enlistee, and later as a Baron, and finally as a Rogue. When he retires, he has three alternate identities he can use, each with their own contacts: his fellow marine buddies and his sergeant; functionaries in the government offices he was in charge of; and the Underworld kingpins he dealt with. He has four valid Imperial ID cards. Imagine him loading a Type A trader with contraband from his Mafia contacts, and then flashing his Baron's ID to the Starport Authority.
Military careers can request a transfer, to one of the other military careers or agents. This required a task roll, I believe, to successfully transfer, and I imagine the number of terms were determined up front. Very official.
Scholars can have one sabbatical, for any length of time, and attempt to enlist in another career. I imagine a waiver or two would be available to such a person.
These systems have interesting combinations. For example, a Rogue could masquerade as an Agent, and could therefore go 'undercover' every other term as an Agent, although when the Agent career was over he would be back in the Rogue career. Very amusing. And an Agent or Rogue could also pose as a Noble for a time, perhaps receiving a Baronial patent (under a fake name, of course)!
Or imagine your PC becoming a Scholar, due to high INT perhaps, and doing some hands-on research (requires a pistol and a whip) for a couple terms, then taking a sabbatical and becoming a Noble for a few terms, crushing some petty insurrections with an Imperial cruiser, getting a Knighthood, then returning to scholarly pursuits.