Orwell wrote: Can I upgrade from the professional membership to developer membership for the different in the price between the two?
As with all our membership subscription you upgrade by paying the difference.
Actually the algorithm is to calculate the residual value left on your existing subscription (based on the purchased price, the initial duration and the duration left) and subtracting it from the new plan price.
So if you purchase for example Professional membership today and the upgrade on the same day to Developer, you will pay the difference (since the residual value is in this case the full price of Professional membership).
Hope this is clear.
nant