>>If you truly understand the OpenGL programmable pipeline you understand graphics programming in general and should have to spend little time picking up a new API. Just like how you should be able to learn a new programming language with ease if you have enough experience.

This is very true. However, I rather not spend anytime on this if I don't have to. What you are underestimating though are the quirkiness of the new API that could waste you hours and hours. i.e. Some graphics rendering may not look quite right because some unexpected limitations, etc. etc. Finding work-arounds for all those tiny quirks can be time consuming Time that you'd rather spend on the product itself.