This has come up before. I don't think you can buy PhotoPaint as a standalone program. It only comes as part of the Corel Draw Graphics Suite. If that is the case you probably can't get an upgrade just for PhotoPaint. God Bless.
I may be wrong, but I think that Corel has it set up like this:
Photo Paint: More basic raster software for photo-manipulation
Paint Shop Pro: Similar to Photo Paint, but more digitally oriented and advanced, like Photoshop.
Painter: Specifically for Digital Painting
CorelDraw: Vector Manipulation
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