Keeping same product URLs after upgrade

When we upgrade our site to 3.0 from 1.5, I take it product urls will be different?
I don't want 100s of external links going to a 404 page after the upgrade.
Any suggestions?

As long as your product and category SEO keywords haven't changed, then the URLs should be the same. If they aren't, the upgrade probably wasn't done right, and the place the SEO keywords are stored wasn't filled in correctly. But if done right, product URLs shouldn't change.