Your right, as if Greg.
If you create a page with five images added using drag and drop stacks, when you replace those five drag and drop image stacks with ones that use warehoused images the original images will remain on the server, in wherever it was that RW put them.
This will not impact your page though, in neither weight nor load times, as whilst these old images will still be on the server the web page won’t use them.
What you will need to be careful fo though is where you’re using an image stack that can handle both local and warehoused images. If you initially set it up to use a local image be sure to remove it before adding the warehouse URL, otherwise in some cases it can publish and load both!