Hi!
It looks like a bit of a problem. Joomla 1.0.15 and 1.5.20 are having a little different address formats. If you migrated your old data to new Joomla version, SEF generated new records for new, a little different, links. This SEF links are probably different from your old format SEF URL addresses. It should be a problem, when you are using SEF URLs in articles and orher content. Becouse this links will be out of order.
There is only one way to change it. Create a batch script, to change your old non-SEF URLs to new format.