It is pretty simple to convert the Carrera digital system to run analog cars, the easiest method only require one switch to swap between the digital and analog powerbase.
Here is a sketch I made earlier on the topic:
Note that the analog "driver stations" represents the analog powerbase, it's just a choice of using a standard analog powerbase, or wiring up driver stations yourself.
When you flip over the digital powerbase (Black Box or Control Unit) and remove the cover plate, then will see how easy it is to add that switch to the cables that goes to the rails, you don't have to mess with the "internals" of the BB/CU.
I took some pictures of the conversion on my track that I can dig up if you need help later.