Manager already runs on 64 bit OS’s. Whether there will be issues with Catalina must await release. If there are any, they will be addressed quickly. But Manager is a browser, even in the desktop edition, so there is a known workaround for major problems that was used temporarily for the last major release.
There is no way to import all your old data. Accounting program structures are too different. The approach is to migrate balances as of the transition date. You can also use batch operations to import things like customer lists after manipulation in a spreadsheet.