The Server/Data Center to Cloud migration of the MultiExcerpt macro can be performed without any loss of service for your macros on your Server or Cloud instances.
The MultiExcerpt macros have different names on Cloud than they do on Server/Data Center. For example, the "multiexcerpt" macro on Server/Data Center is "multiexcerpt-macro" on Cloud. In addition, the macros have a few parameters whose names need to be changed.
You will perform a migration with no loss of service on your Server/Data Center by enabling app/macro modules (on Server/Data Center) that use the MultiExcerpt macro names for MultiExcerpt-Cloud.
Enabling these app modules will allow you to rename your MultiExcerpt macros (on Server/Data Center) by editing them in your Confluence Storage Format XML (SFXML) representation of your pages. During this renaming process the MultiExcerpt macros will work on your Server/Data Center while using either the Server/Data Center or the Cloud macro names. After the renaming is completed for all pages you can migrate your pages from Server/Data Center to Cloud. The pages are immediately ready for service on your Cloud instance.
Note: there is no Cloud equivalent of the Server/Data Center macro for MultiExcerpt Enhanced Permissions so you will need to think about your strategy for Cloud excerpting of restricted resources before you do this migration.