Trying to get upgrade pricing for LINQPad 6

Hello,

I have a license for LINQPad 5 (premium edition) and I am looking for pricing for upgrading to LINQPad 6. I got to the "Custom Upgrade" page, and then get the message "Code 'XXXXXX' has been upgraded. Please use your new activation code." It just spits back my original code that is not valid for LINQPad 6 (I checked and after doing this it still says the code is not valid for 6). Maybe this happens because I already upgraded once and that means I need to buy a full license?