The switch from 1.2 to 1.3 was quite some time back, so I can't say from memory how exactly updating was recommended back then. I think we had a reminder in updates that said "Go to the website and download the new version", but the problem is that that appeared only once, and may have been clicked away by some users.
Detection updates are still available to older versions, and would download there, but they're probably not working 100%, missing some things... in which case Spybot-S&D 1.2 should tell you that some detection rules would need a newer version - unless you disabled that warning in the settings
By the way, 1.5 should be compatible again, and would already be ready if we wouldn't have these lengthy discussions with Microsoft about their requirements for security software to be more unsafe then necessary to get Certified for Vista Will probably post something about that soon