Custom tags will lose when change task type

I have a problem when I try this example:

when I change the task type from bpmn2:task to bpmn2:userTask, the custom tag qa:analysisDetails will lose in userTask, I want to retain the qa:analysisDetails tag, what should I do?

Replacing an element through changing its type will always result in the extension elements being gone unless you add a module to bpmn-js that copies those on moddleCopy.canCopyProperty which is an advanced use case.