Plans are on an annual base. When you start the plan you already chose then plan based on the maximum amount of employees your company has.
If you have hired more employees and you need to upgrade your plan because you have already reached that limit, you can upgrade it at any needed time. In that case, you will pay the rest of the difference for the remaining period for the upgraded plan.
If you had to fire some employees, we won’t be able to change your plan. There is no option of downgrading a plan. You can always though change it in the lower plan when it will finish at the end of the year.