registration

On my computer I had an old version of Little Snitch, that always asked me to buy it. Yesterday I bought it without paying attention which version it was. When I entered the registration number it didn't work. I found out, that I bought Little Snitch 3, although for my old system of Mac 10.5.8 I would need an older version of Little Snitch.But the registration number does work only for Little Snitch 3! How can I change it?
Thanks!