Change 4C Grey/Black to Key only

[Istvan22067830dxdt]

I'm currently facing a problem that I've faced time and time again. We got a print file from a customer that's in Pantone colours and different shades of 4c grey. However for the file to be printable with our supplier, the black needs to be in key only. 

With some files I managed to get the black to be Key only in the color converting options, but this only works about 20% of the time for some reason. 

Does anybody have a solution for this? I need to preserve the 3 Pantone colours in the document as well as the gradient.

[Eric Dumas]

Is there a chance you can get the original illustrator/InDesign file?

[Istvan22067830dxdt]

In this case I actually did and exported the file myself. However we have a lot of cases where all we get are the pdf files and it would bre great if I found a solution for this in the future.