From the Adobe site, it appears that if you own ANY version of Lightroom, you can upgrade to Lightroom 5 for $79. I imagine you need to remember your key - but don't know that.
EDIT: Actually can get the upgrade for PC/Mac at Amazon for $75.
Posted by Rick Lamb on September 5, 2013 at 1:51 AM
You will indeed need the registration key from the previous version of Lightroom to execute an upgrade...
Discussion
2 repliesuse code S987654321 when checking out.
http://www.adorama.com/ABLRV5MW.html?emailprice=t&j=Email042413New&utm_term=Other&utm_medium=Email&utm_campaign=Email042413New&utm_source=RSYS
EDIT: Actually can get the upgrade for PC/Mac at Amazon for $75.