You ask the flight office. They contact Crew Skd and they review the manning situation.

Dropped days automatically gets replaced by vacation days from next year. But a vacation day is worth :48 less than a reserve day. If you have vacation hours left in this year they subtract 48 minutes per day dropped. If you don't have any vacation hours left from this year your guarantee drops by 48 minutes. That's my understanding of the system which might be wrong.

If you don't want to use vacation days you can call the pay folks and ask that your guarantee gets reduced instead.