You may have received an email from the “Lotus Notes Migration Team” warning you to “avoid creating or renaming folders until you receive your Welcome message”. The reason you were sent that message is because your Lotus Notes mail file is being migrated from your on-premises mail server to your new mail server in the cloud. While that migration is proceeding, there are two copies of your mail file – the copy being migrated and the copy still on your on-premises mail server, i.e., the one that you are using now.
The copy being migrated is being processed in a number of ways. It will, among other things, have design changes applied to it. When you create or rename folders in your on-premises mail database, you are also making design changes.
If you take any such action (for example, rename a folder) during this transition period, your change will be in conflict with the design changes being made in the migration copy of your mail database. When migration is complete, the two databases will be replicated with each other one last time. The replicator will discover that there are two versions of your changed folder and delete one of them. If your version of the folder gets deleted (50% chance), any documents that you placed in the folder during the transition will be orphaned. You will have to locate them in the All Documents view of the database and re-folder them.
To avoid this problem, your best course is not to create new folders or rename existing folders during the transition (which should only last two or three days, tops). If you have already done so, you might want to make an independent record (for example, take a screenshot) of the added contents of the changed folders so that, after the transition, you can more easily locate and re-folder any documents that become orphaned.