Using Office 365 Sensitivity Labels

Well, you can search for all the documents to which you have applied retention labels to and remove them (search for compliancetag not blank), but you can't replace them with sensitivity labels just yet (at least, I haven't found a way). But we're only starting with sensitivity labels and I wouldn't do anything just yet until we a) see how they are integrated into SharePoint (what metadata is used) and b) how they are integrated into the Office Online apps and the SharePoint/OneDrive browser apps. Remember that a document can have a sensitivity label and a retention label, so maybe you'd leave the retention labels in place and work out how to use the sensitivity labels most effectively in your organization when they are fully baked.