I upgraded a while ago and have been opening Motion 2 projects with no problems. It will ask you whether you want to convert it to v3 or open it as a new Untitled project, preserving your v3 original. Generally I avoid upgrading in the middle of a project, so hold off if you can (especially if you have older FCP projects you're still working on), but upgrading Motion projects doesn't seem to be a big deal.
JK
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SCQT! Self-contained QuickTime ? pass it on!