Replication must be up-to-date before transferring operation master roles. If replication has not converged, then several symptoms may occur. The role may take significantly longer to transfer, or it may not transfer at all. Likewise, the new operations master may not receive changes that were initially sent to the original operations master. This could result in an inconsistent Active Directory. Thus all replications must be completed before beginning the process.
Check the replication status first. If necessary, follow the article below to resolve any replication issues.
Active Directory Operations Overview: Troubleshooting Active Directory Replication Problems
Once replication has synchronized end-to-end, and Active Directory has converged, the roles can be transferred. Follow the article below to transfer the role in question.
How to view and transfer FSMO roles in Windows Server 2003
The risk of Active Directory becoming inconsistent is higher if the role is seized rather than transferred. For that reason, Microsoft recommends you let a full replication cycle elapse between any changes before attempting a seizure.