When a modulation uses the old legacy envelope you cannot add other modulators. After removing the legacy envelope there is no way of upgrading and it will not be possible to either add new modulators or upgrade the chain.
Double click to add a device will ask to upgrade, clicking on another legacy envelope in a different target within the insturment should still work, and right clicking the instrument and choose upgrade should also still work.
You are correct! I was brought to a dead end by my habits and what I expected. I thought the upgrade button was the only way.
(Maybe should ask to upgrade when trying to drag modulator to chain? Anyway, it won’t happen again now that I know how to do it.)