Hi Glenn
The type XIV is indeed referred to as the "Milk cow". But I'm not sure it is a converted type IXD, to be honest.
At first I thought that the type XIV must have been derived from the type VIIC. That opinion was mainly based on the lengths of the U-boats I looked up on uboat.net. The length of the type XIV is identical to the length of the type VIIC, not the type IXD (which is actually 20 meters longer).
Not completely satisfied I looked it up in the book on U-tankers from Axel Urbanke and there it actually reads that the design of the type XIV was indeed based on the design of the type IX boats.
Note however that the U-tankers were shorter (9.4 meters) and considerably wider (3 meters) than the type IX(C).
Sounds like a whole lot af scratchbuilding to get from a type IX to a type XIV. On the other hand, who would ever have thought that Revell would be releasing a type IX in 1/72? Who know that somewhere in the future they might even release a type XIV? Hope never dies, as they say...
But if you're impatient, I'm sure you'd be able to make a type XIV from a type IXC looking at your scratchbuilding skills !!!