Not usually, no. The job you sign up for is the one you should expect to keep.
Army roles (engineers, artillery, logistics, infantry) are called ‘cap badges’.
You sign up for a specific cap badge and once you’re in you don’t have a right to change it.
We get a lot of emails from soldiers saying they thought they’d be able to change their cap badge after they’d signed up (or after they’d finished training), only to be told afterwards that they couldn’t.
Now they’re stuck in the army in a job they don’t like and can’t do anything about it.
Be careful, no matter what you may be told, you have no right to change your cap badge once you’ve signed up, or after you’ve finished training, or at any point.
If you sign up for, say, the infantry, you should expect to stay in the infantry for your army career – whether or not you’ve been told otherwise.
There are exceptions but they’re very rare.
If you do sign up for a cap badge and find you don’t like the job and you still have the right to leave, then you have the option of leaving the army and re-joining in the new role.
If the army says that you don’t need to do this because they’ll change your cap badge later on – but they won’t put it in writing for you – then it really doesn’t mean anything. You can still leave anyway (but only while you have the right to do so) and re-join in the new job if there are spaces and if the army wants you (i.e. if you haven’t caused them any trouble).
Lots of people get caught out by this, so be careful!