Currently CCMA does not support migration of spaces which are already present in cloud, we first need to delete them and then migrate the space.
It would be hepful if we can have differential migration feature in CCMA. Have a migration saved in the source system, then click a button and the CCMA would take the page diffs, find the corresponding pages in the target system, and just apply the changes. This helps because oftentimes, users keep working in the source systems even as we do the initial migration.
Also, we should have an option in CCMA to provide an updated space key when we don't want to merge with the space already existing in Cloud.