It's possible for existing governors and trustees to change to a different category mid-way through their term of office. This is as long as they're eligible to be appointed in the new category and the right number or proportion of governors and trustees in the new category is maintained.
Before making the change
Make sure you have a good business case for the change, aimed at keeping necessary skills on the board. Rather than getting into a cycle of appointing the same people repeatedly, you should carefully consider the needs of the board.
If you're in an academy trust, check your articles of association and schemes of delegation for any specific rules on changing category and to understand who can make appointments.
The first step is for the