There are two straight-forward ways to achieve this. Either you convert an object to the new type. The downside with this is that you will loose the old object. Another way is to first create the new object and then replace all references to the old object with references to the new object.
To replace references to an object, right-click the object in any of the windows displaying it. Select Find/Replace references from the menu (the option might be hidden under the sub-menu Apply).
A window will now appear showing all the equation references to the selected block. To replace the references, click the button. A panel showing all the objects of the current project appears. Select the new object from the panel and click Ok