Can I upgrade HueForge for less? Or do I have to pay twice?

NOTE: If your market has alternate pricing, you must add a D in front of the coupon code.  So PUUPGRADE becomes DPUUPGRADE

Absolutely you may upgrade your version of HueForge.

Do this by adding the tier you want in the shopping cart, then apply one of three coupons:
PUUPGRADE - use this if you currently have Personal Use
LCUPGRADE - use this if you currently have Limited Commercial
PROUPGRADE - use this if you currently have Professional.

Each coupon code can be used once, so you could upgrade from Personal Use->Limited Commmercial
Then Limited Commercial->Professional 
Then Professional->Lifetime Professional.

However if you upgrade to Limited Commercial, you cannot then use PUUPGRADE again for your annual re-enrollment.  Or against any other upgrade.  And the LCUPGRADE and PROUPGRADE coupons are good for the period during your upgrade.

Upgrading from Limited Commercial->Professional restarts the clock on your year.