Can we plaster our ceiling without removing the crown molding
Yes, it is more likely the crown molding would be damaged by removing it to refinish the ceiling than by leaving it in place (see Should I remove moulding before adding a plaster ceiling? and Can I install drywall on the ceiling, without removing crown molding?)
It's fairly easy to leave it and tape off (or tape-and-plastic) to protect the molding while refinishing the ceiling.