You can do this using Adobe Acrobat Pro (Note that this is not possible with Standard version – Adobe Acrobat Reader).
1. Open up ‘Convert colors’ from ‘Print Production’ tool bar on the right side.
2. In the convert colors dialog, select ‘Preserve Black’ in the ‘Convert options’ group. Change the output intent if required and click OK. This will convert all colors that are rich black to 100K.
You can adjust the target objects that you want to convert – for example just text; the default changes all objects.