The simplest option is to create another instance of n8n with latest version and move your old workflows there one by one (with copy+paste or via export/import)
hell no. Completely new instance with latest version of n8n and with new database.
The old versions (before v1.0) has some nodes which does not exists in the new one or they have been replaced. Plus there were many changes in the DB also.
Once you have moved all workflows into the new instance, you can decomiss the old one