If your average hours in a twelve week period is less than 34 hours, than, yes they can bump you to part-time.
It's the part-time/full time exception report.
So if you are taking unpaid time off it will lower your your average hrs per week.
The only time they won't move u to part time is if you are on an LOA.
So if you are going to take time off as unpaid it's totally up to you, but it may affect your status.
They cannot stop you from taking time off if you are full time as unpaid.
Just know it may affect your average.
I hope this helps...