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301 will pass link juice all day long my friend. URL rewrite... not so much.

I can say from personal experience "inheriting" a bad code base or architecture and failing to correct it for the pain or cost of doing so makes everything else that much harder to do. In this day and age there is no excuse for not simply tackling the problem and doing things "right" . Changing CMS and blog and converting nearly 8000 pages to them was tough and took nearly 3 months of continuous work by 2 full time employees, but the rewards far outweigh the costs and pain.