Refresh into devmode

I have a strictly confined snap installed from the store which I’m trying to debug. I have installed it, and setup some data, now I want to switch to devmode to see if confinement is indeed the issue I suspect it is. There seems to be no way to do the equivalent of:

snap refresh foo --devmode

The only way to switch to devmode seems to be to remove the snap completely (losing all my config and data) and re-installing it again. Is this by design or a bug?