You might find this thread helpful.

http://forum.makemusic.com/default.aspx?f=5&m=474172

I'm not sure what you did, but here's a method I found while messing around a little:

1) select "Staff Tool" > Select "Staff" in the top menu > choose "Define Staff Styles" >

2) create a "New" staff style > under "Appearance" select "Settings" for alternate notation > choose "Slash Notation" (Large Slash) or "Rhythmic Notation" (Small Slash) > select a layer and other settings you want this applied to >

3) select "Document Options" from the top menu > go to "Alternate Notation" on the left pane > where is says "Music Characters," choose between "Large Slash" or "Small Slash" (whichever one you picked when defining a staff style) and then "Select" a glyph to use.

4) once you're done with that, "Apply Staff Styles" (located just under "Define Staff Styles") using the "Staff" menu to the measure or measures you want.

There should be an easier way to do this. You may want to post in the feature requests section of the forum to see if something can be done about simply changing the noteheads in a given layer.

Thanks George, I'll give it a try

You could choose Layer 3 and then use ALT-SHIFT+S to show only that layer. Then choose the entire staff and use Utilities > Change > Noteheads to change all the noteheads to whatever you want. 

Click ALT-SHIFT+S again to toggle back to showing all layers.

(Then change your layer back to whatever is customary.)