I was under the impression that, to convert the colour mode of an InDesign document, you just had to modify
Edit > Transparency Blend Space and change the colour mode of all swatches.
But I've recently realised that the default
Black swatch will stay as it was when the document was created... and of course, you can't modify it in the way that you can with other swatches.
I suppose you could create a new 'black' swatch and set all blacks to that. But it would be a little tricky to select all of the default blacks to change them (maybe you would miss some).
Is there a more feasible solution to this without having to create a new document?