Here are some suggestions for resources on converting journals from subscription based to open access.
CAUL’s Fast Track to OA Toolkit for Publishers
Society Publishers Accelerating Open access and Plan S (SPA-OPS) project
SPARC – Transitioning Your Journal from Subscription to Open Access
This paper, Attracting new users or business as usual? A case study of converting academic subscription-based journals to open access, describes a case study of a group of Norwegian journals that transitioned from subscription to open access.