Hi @pinguin,
For migration from 6.7 to 7.x you can set migration.6_to_7.enabled: true before upgrading to 7. This should help with your dashboards, I'm not sure about your correlation rules (the field aliases the setting creates work for queries, but not for the actual document contents in _source). See also the documentation.
I would recommend keeping the events in ECS. There might be some inconsistency between 6.x and 7.x events (7.0 is the first release that had ECS) - but going forward we will keep ECS in place across data sources.