Changing Timestamp while preserving permalinks

Hello, I recently migrated from typepad to wordpress and I’m thrilled with WP. However, there is one snafu that I’m having.. For my particular weblog, I often take old posts and update them, change the timestamp and post them as new. Everything works fine, except the permalink for the old post now longer resolves to anything(since the permalink is date based). On typepad and MT, even though you change the date, a copy of the post is preserved with original permalink intact. On WP, since it’s database driven, there is no “old copy”. To make things even more complex, I imported tons of my old posts from Typepad, so I can’t really change the permalink structure.

Any ideas?