replace maps in purchased adventure without breaking anything?

I bought Dungeon of the Mad Mage and want to use most of it, but the maps are very bare bones.  There are nicer looking maps available on various sites.  How do I swap them in? When I tried pasting one into the map layer on top of the existing map and started rescaling it to fit, it did weird things to the dynamic lighting so I ctrl-Z'ed backwards and deleted it.

The dynamic lighting is drawn for the specific map. If you find a map with the exact dimensions  and use that as a replacement, then you won't have to make any changes to dynamic lighting. In most cases, there are significant enough changes such that you have to redo it from scratch. I've had to redo the entire map for the Sunken Citadel, for example. Unless you were talking about some different weird things.

Go up to the Pages menu, Create a new Page.  If you want the same grid-settings as an existing Page, or even the same Dynamic Light lines, you can do that with "Duplicate Page" in the same place. Click onto your new page, and Paste or Upload your new map there. If the old dynamic light lines don't match up, you'd have to delete them and re-draw them, depending on the shapes in your new underlying map.   If it's really the "same" map at a different scale, we might be able to re-use the Module's existing dynamic lighting lines, if we can resize/scale the lines to match perfectly enough.

I don't know if this is also true under the current DL system, but at least under Legacy, scaling DL lines could lead to unexpected results. But it's definitely worth a shot.