SymptomsParallels Panel does not suggest to choose dedicated IP address of the domain during migration. It could happen if you migrate mail content of the domain separately from hosting content as described below:
A domain is hosted on an exclusive IP address. It is migrated to another Parallels Panel server using "All configuration and content except mail" option. An exclusive IP address is assigned to the domain during migration. If later an attempt to migrate the same domain using "Only mail configuration and content" option is made it will only be possible to choose different exclusive IP address to host mail subsystem of the domain.
ResolutionMigrate mail content of the domain, select another exclusive IP address and complete the migration. New exclusive IP address will be assigned to the domain after the migration is performed.
Change this IP address to the desired one manually.