You might come across an apparent issue where trying to substitute a RealDrum style with another in a song. Although the Drum track in the song style is updated with the new selection, it still doesn't play it. I experienced this "enigma" with the _FREENOW.STY when I wanted to swap the original RD style with the RD "AmericanSlow8thsFred.STY"

PG Music support provided the cause and solution as follows:

The reason that the RealDrums substitution is not working is due to the fact that the style is using Multi Drums. This is a collection of RealDrums styles all playing on the same track. It involves a few steps to remove Multi Drums since they are embedded in this style. Before you proceed please note, once you remove the Multi-Drums and save a style the style will no longer have Multi-Drums any time you select it from now on.
1. Go to File | Style Maker | Edit Current Style
2. Press the Misc. button to bring up the Misc Style Settings.
3. In the bottom left corner press the button Set MultiDrums.
4. In the Create Multi-Drums window press the clear button and then OK.
5. Press OK in the next window and then press the Save and then the Exit button.

Now you should be able to substitute the RealDrums you have selected into this style. This will not happen with every style or RealDrums, only the ones with Multi-Drums.