ProMods

I've done a few searches for this but can't find an answer, so I figured this is the best bet for an answer...

If I edit a map (in this case changing/adding roadsigns across the USA map) and SCS later updates the base map, can I apply my editing to the updated map without starting from scratch?

Your sectors always overwrite the base map.

Yeah that's what I assumed. But is there any way to copy what I've done on the edited map and apply that to the updated map?

Well... If you do something and save it, and then load in game, it'll be applied to the map.
If SCS updates the map, your work will still overwrite it. Every map mods overwrites the original map because it loads after the original map.

That's what I thought but I was hoping for a different answer. Would be handy if there was a 'layer' feature in editor.

Layer? How do you understand it in this case?

You have kind of a layer system as in a base file with all the roads, terrain and functionalities and an aux file with all the buildings and models. But trying to implement a fix SCS has made in a sector you edited will require some "Frankensteining" as we like to call it.

And can I take it that when you say Frankensteining, the process is not easy and may lead to...undesirable results?

It basically means loading one version of the sector, copying the changes made there and pasting them in the other version of that sector. Which can be quite fiddly.

I understand the process, but not the how to. And believe me I am interested in the how to.