The ability to easily replace style x with style y has been on my To Do list (on my Trello board) since June of 2015.
After some screen estate jiggery-pokery, I made some room for the function on the Styles tab within the main Toolbox. The Total label and field have been removed from Styles, Breaks, Comments and Bookmarks and instead now appear in the main frame’s title on each tab.
You now see two drop-down boxes which list *every* available style in your document. Select the old style, e.g. Heading 2, then select a different style in the other checkbox and click the Replace button. Depending on the size of the document and how often the old style is used, the style replacement is done as quick as possible.
The latest beta version can be found on the usual DropBox links.