I had a cousin who was a butcher and cook for troops not far behind the lines. When the Battle of the Bulge began, he suddenly became an infantryman! (Survived.) So did a lot of the black Red Ball Express guys.
One of my uncles by marriage was a motor vehicle maintenance officer in a QM truck company. They were occasionally on both the Red and White routes, having been in France and Belgium since August of 1944. They routed trucks full of supplies to the units fighting in the Bulge, but they were unloaded and turned around fast, the drivers going like hell (more so than normal) back and forth. My uncle remembered the trucks going back all covered in frost and such, and pretty much beat to hell by the constant pounding they were taking.
I don't think any of his drivers were converted to infantrymen, but some of them did come back without trucks.