We are using SharePoint 2016 and are going to migrate to SharePoint 2019. After the migration all the existing sitecollections will still be in classic mode. We are using the classic team site template. How can I migrate all the existing classic sitecollections to the modern experience team site template?
Their is no out of box process available for this conversion. Last time when i checked their is no 3rd party tool to which support the conversion & migration of Classic to migration.
another work around, you have to manually move the data from classic site to modern site.
you can make your current site to looks like modern using the new pages, creating new modern list/lib etc...you can get the idea by the going through this Microsoft link: https://docs.microsoft.com/en-us/sharepoint/dev/transform/modernize-classic-sites