It is definitely possible to switch roles at a studio, but it is not necessarily easily accessible. Generally, we hire people for one specific role (e.g. tools programmer) because we have a lot of work on a specific project for that specific role - we might really need a tools programmer to get this game out the door. Once the project ships (or significant progress is made) and the necessary work is done, opportunities for promotions and transfers usually open up.
If a tools programmer wanted to switch tracks and become a gameplay programmer, what would normally happen is that the tools programmer would speak to her manager about the direction of her career, usually in a one-on-one session. Her manager would note it down and they’d discuss what it would take in order to switch roles - whether a gameplay role is open, whether the tools programmer has the necessary skills, and (if those skills are lacking) how the tools programmer could learn those skills. In situations like this, the tools programmer would start taking on some smaller gameplay tasks in addition to her normal tools work in order to level up her gameplay skills. Then, once the tools programmer had leveled up sufficiently and a gameplay role opened up at the studio, she could make the switch and they’d open up a spot for a new tools programmer to replace her after she made the switch.
There are a lot of side effects to switching roles. Sometimes, the role change would require taking a pay cut. This often happens if an engineer wishes to switch to a design role. Sometimes the role change would require a team transfer - the studio might have multiple projects in the oven and the opening for the desired role would be on another team. Sometimes the role would require moving, though this is less of an issue with the rise of remote working. Back in the day, however, taking on a new role (be it a lateral change or a promotion) would sometimes involve having to relocate to a newly established studio location. If you’re thinking about it, talk it over with your manager. One of the responsibilities of managers is to help their reports achieve their career goals.
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