You're probably better off using third-party tools, as Microsoft doesn't natively offer this functionality. What they do offer is two sets of APIs. First, the "standard" Graph API approach, via the chatMessage resource. There are certain limitations to this however, such as being able to preserve timestamps and original author information. A different approach is based on the Import API, but that's technically not supported for Teams to Teams migration: https://docs.microsoft.com/en-us/microsoftteams/platform/graph-api/import-messages/import-external-m...
