I could be wrong, but I think the Downgrade User button is to downgrade a user from a Named User license to a “regular” concurrent license user (users sharing the concurrent licenses upon login).
To my knowledge, ECM does not have a View Only license . . . unless that’s a special offering for companies that have ECM and Epicor ERP???
So I bumped into the answer for this looking for something else. Under manage there is a heading View Licenses which I thought would let me see the modules I was licensed for. But it is actually your View users maintenance which had an Upgrade User button. So I made a view user and then upgraded them and the user did in fact show in the normal users screen and then I downgraded them and the user “disappeared” back to the View Licenses list.