To transform an entity, changing the entity class than to something other than the original and have all the shared attributes copied over, focus your attention on the toolbar. Find the dropdown labeled More and click on it to expand it.
With the More dropdown open, select the option labeled Transform and the dropdown will expand to reveal your choices to transform the current entity into.
From the options listed in the dropdown, make your selection of entity class, then click on the associated label. Then the page will refresh to the newly transformed entity, with all the attributes that were previously existing present if they were compatible.