I'm going to answer publicly to a question I have receiver by private email but that may be of interest for all users of the plugin. The question is:
We are going to do it in two steps:
We use post-function Copy parsed text to a field to set virtual field "Ephemeral string 1" with the name of the priority we want to use:
We use post-function Update issue fields to set virtual field "Priority" with the value stored in "Ephemeral string 1" in all issues returned by the following JQL query:status = "In Progress" and status was "In Progress" before -1d
Once configured, the transition configuration will look like this: