Sorry to not get back to you very quickly. We were travelling the last week, so the free extensions didn't get the normal level of support that we normally shoot for. I hope your migration went well.
We removed the releases prior to 1.5.10 because the zip file was over 2mb, which is a limit on some servers.. so we made a judgement call to cut it off at that point. Unfortunately, this affected you.
If you have any other questions or feedback about the process, let us know. We'd love to know how your migration went, and how you're liking Joomla 1.6!