I have gone through this process, and yes you have to delete the oxid6 database, then import your old database into oxid6 (now emptied) database. then run migrations.
Then after that delete the configurations of the modules from database with this query
delete from oxconfig where oxvarname in ('aModulesHistory','aModuleVersions','sUtilModule','aDisabledModules','aLegacyModules','aModuleFiles','aModulePaths','aModules','aModuleTemplates');
Or you can just simply follow this link: Admin Panel and Shop Frontend Giving localhost 500 error