Already own a paid edition of LINQPad? This page handles every kind of upgrade:
You can also use this page to check what kind of license you have.
The above offers combine with the current discount on Premium editions.
Your old activation code(s):
Use spaces to separate multiple codes. You can mix and match old and new licenses.
LINQPad will calculate an overall trade-in value and your new license will combine everything into one.