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

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.