UPDATE: After playing with Gimp and Paint.net, Gimp turned out to be simpler than I expected once you get the hang of the various capabilities. The initial part of getting the images lined up isn’t as hard as I expected, being really much the same as doing it in GE, you don’t need to know about alpha channels and masks and things to get to that stage. Paint.net isn’t quite as easy to use at this level in my opinion, or maybe the loss of functionality is the issue. After trying both I have settled on Gimp for now, although admittedly having it on MainPC is easier than working on the Windows 10 computer with its smaller screens that aren’t straight in front of me.
Of course once the hard part of lining up the images is done then a bit more learning would bring me the capability to paste in the old riverbed into the current aerial imagery and then be able to draw the maps over it more fully. I expect all that is needed for this is to be able to turn the current relatively smooth riverbed into a selection and then merge the old aerial layer into it. I am sure there is a name for this in Gimp/Photoshop jargon and eventually I will learn this.
However having the time to do this is crucial and it’s probably going to have to wait as I have plenty to keep me busy for the present.