Aha ok, I see! After a quick test, however, its only possible to downgrade the way to "mothballed" when CTRL-dragging the mothballed way over an existing piece of way. I understand the safety mechanism to prevent accidentally downgrading ways, but as there are no waytypes more bad than the mothballed waytype, it does not make sence to have to press CTRL.
I will arrange your description to fit in what I already wrote!
Just curious, feel free to ignore if everyone already knows (the little Simutrans I play isn't Experimental - although I love keeping up with all the awesome stuff that happens with Experimental, mind); but if the way is mothballed with a train on it, would it get stuck? Just passing curiosity from me - please don't expend much energy on an answer. 
It once happened in my test game that a train got stuck on the line on a mothballed way (I disabled the auto upgrade to test) and I remember having some struggle to get it going again. However, it should be possible to "upgrade" the way again underneath the train in order for it to continue!