Did you happen to use your forum email for your Cake Store Account/registration?
If so I think you'd just need to log into the main Cake site with that and navigate to your account page.
Might require you to "merge" your accounts (that weird crap we all had to do a few years back).
If you used a different email and don't have your Cake account password handy but remember the email then you could probably use the "Forgot Password" option to get in there.
Once you do get into your account your registered products should be in there and you will have access to upgrade pricing (if not then yes... you'll have to call Cake and have your product info).
That said... that is an oooooooold version you got so maybe the whole account/registration process was different back then. In which case, again, a call or support email will be required... with your product info.
Essentially, you need to get into your Account/Registered Products page then you'll be golden (unless you are one of the unlucky ones who's account didn't updated properly when they rekernoodled the site).
Hopefully that helps/makes sense.