For those among the Corel Users who still use Photopaint X3 or older versions: you can get your nib preview working properly, no matter if you delete nibs or change the default nib library.
If you ever have deleted a nib from your nib library you mostly use, you might have made the experience that the preview shows confusing preview icons. The icons are reduced by the number of nibs you deleted, but they don't seem to update. The same problem occurs when you change the nib library or append nibs. The preview seem to maintain the 'old' nib preview icons. The result is very frustrating, cause you can't depend on the preview anymore if you change the nib when painting.
Here is a solution, which works.
In a folder on your local drive (commonly C:), there are two
.pst-files: BrshIdx.pst and BrshImg.pst. They store the preview icons
of your nib (don't confuse them with the .msk-file and the .nib-file,
they store the actual brush nib data). Those two .pst-files are once in the
directory C:\Programs\Corel\CorelDRAW Graphics Suite
X3\CorelPHOTO-PAINT\Presets\Brush, and are the origins.
Twice they are in the directory C:\Documents and Settings\Your User
Name\Application Data\Corel\CorelDRAW Graphics Suite X3\User
CorelPHOTO-PAINT\User Presets\User Brush, these are copies of the origins in the Programs path. They are updated when you have exit Photopaint. But to get your Preview working properly, you have to put hands on it.
Close Photopaint, after you have changed your nibs.
Select the two .pst-files in the User Brush folder and Copy them (Ctrl + C).
Switch to the directory of the Brush folder under the Programs path.
Paste (Ctrl + V) and owerwrite the two .pst.files.
Now your preview of the brush nib should work fine now, and there are no barriers anymore to organize specific nib libraries and switching between them.