I have been using LR for several years on my mid-2011 iMac and have approximately 26,000 photos. Have upgraded to a new iMac (10.10.1).
Considering switching to the cloud version now or just migrating my software version to the new computer. Migrating is kind of complicated with lots of files, catalog, etc. but it seems like the most cost effective way to go. In principle you are supposed to start with the latest version of LR, and it is not necessary to bring over the old one. What to do????