I do this every few months with my older pre-vudu collections. Stupid me threw out the digital copy codes that required you to put the disc in, register the disc and get the code to view it on your computer. Got it to work ONCE with one of the Family Guys Star Wars movies but it was a PITA. So I gave up.
Yes there are certain movies that it won't let you convert...for example the Star Wars Saga or any of the early marvel films starting with Iron Man 1. At first The Wolverine unrated was not featured for digital when it was first released but now it is so I will have to convert that as well at cost. I do notice that it will convert the movie only so you won't get the special bts features. It did however upgrade my old ID4 copy to the remastered without being charged.
Walmart use to give out a LOT of vudu 5 dollar codes with their pizzas but I think people were stealing the codes (since they were exposed) and using them without buying the pizzas.
Don't know if the Indiana Jones movies can convert either.