This is a tracker task to figure out testing / qa needs and any work / changes needed to VisualEditor to switchover from Parsoid/JS to Parsoid/PHP.
T229015: Tracking: Deploy Parsoid/PHP to production is the tracker task for deployment. But, we don't anticipate a deploy before mid-September 2019 at this time. But, let us handle this as if we are going to be deploying around mid to end September.
Since CX talks to RESTBase, there is nothing that will change on that end. However, T229025: Add ability to RESTBase to partition client traffic to Parsoid/PHP and T229018: RESTBase should be able to store Parsoid/PHP contents in Cassandra alongwith Parsoid/JS contents implies that CX might have to handle cookies tracking which version of Parsoid generated its HTML unless we come with a mechanism for handling it in RESTBase / Parsoid that is transparent to clients. But, for now, it is probably safe to assume that CX might need to handle a cookie related to this.
We will also do an early deployment to the beta cluster so that clients can do early testing there. But, please comment on the ticket / edit the description adding any other requirements to ensure we cover all our bases.