"Changing the cylinder is straight forward, so surely the barrel would have been changed as well?"
The barrels used on the Model 58 varied even greatly from the barrel used on the Model 57, also a .41 Magnum. The 58's barrel is heavier and thicker overall and has no ejector rod shroud, just a small lug to house the forward tip of the ejector rod assembly and it's latch. The top rib is thinner and configured differently and the front sight blade has no base.
The Model 58 was the only N frame barrel to use this barrel configuration. There was no production eqivalent gun in .44 Magnum.
It would appear this gun was accomplished by either replacing the Model 58's .41 cylinder with a Model 29's .44 cylinder, or by rechambering the smaller to the larger. Then, the Model 58's barrel was removed and had the bore bored out and re-rifled to .44 Mag dimensions. Changing the markings on the barreel would have been a simple matter.
So, in a nutshell, the cylinders could be swapped out but the barrels would have to be modified.