You will be forced to revert from the plus variant to the standard number of working hours, if you have more than 80 holiday hours left at the end of the year (on a pro-rata basis). This means that, as of January, your contract will be converted into a contract with the standard number of working hours.
You will only be able to deviate from this regulation if you have made a separate agreement with your supervisor, by which you will agree to take time off in lieu of these hours within six months. You will need to ensure that this agreement has been set down in writing. You will also need to notify your personnel HR officer.
If you have less than 80 hours left at the end of the following year (on a pro-rata basis), you can consult your supervisor about whether you plan to revert to the plus variant on 1 January. Make sure that you discuss this with your supervisor in good time.