We are currently running a standalone installation of NC 18.0.3 on Ubuntu server 18.04.4 LTS with MySql Server.
Is there a migration procedure to go from stand alone to an HA setup or will this require a full rebuild from scratch?
If you can migrate the data from mysql to your HA setup database solution, it should be possible. You can change the location of the data folder. However, I don’t know if you could easily move from local storage to object storage or something like that.
If you look at some old deployment recommendations, you have to move all the services to dedicated machines and then put load balancer in front of your current setup: https://docs.nextcloud.com/server/11.0/admin_manual/installation/deployment_recommendations.html