Let me jump in with the suggestion how you could resolve your problem with existing campaign which can't be migrated to AcyMailing 8
Both version 5 and version 8 can coexist on the same site.
Configure AcyMailing 8 as your main newsletter extension, by replacing all the subscriptions forms from version 5 to version 8
Configure also version 8 cron job to use our cron job service
At the same time, configure your own cron job on your own server to trigger a cron job URL of AcyMailing 5 installation
This will allow you to process to send all your queued campaigns from AcyMailing 5 and at the same time use new version for new newsletter handling
There is one thing that you will need to manually update, and this is possible unsubscriptions from the campaigns sent with AcyMailing 5. You will need to manually unsubscribe the same users from AcyMailing 8, since there will be no sync between AcyMailing 5 and AcyMailing 8 subscribers
Hope this helps?