The Flip4Mac upgrade path always gives you credit for what you've already paid.

To upgrade from
WMV Player Pro to WMV Studio - you pay $20.00
WMV Studio to WMV Studio Pro - you pay $50.00
WMV Player Pro to WMV Studio Pro - you pay $70.00
WMV Studio Pro to WMV Studio Pro HD - you pay $80.00
WMV Studio to WMV Studio Pro HD - you pay $130.00
WMV Player Pro to WMV Studio Pro HD - you pay $150.00