I am migrating a site from wordpress and need to preserve my url structure whilst supporting native pathauto urls in the future (for new content)

Overall Drupal's clean url process is almost identical to wordpress but I am finding some small discrepancies and with around 2k posts to migrate over I need a programmatic way to do this that won't feel like I'm trying to roll a keg up hill.

I was thinking of inserting in the old wordpress url as field_legacy_slug and then I could create paths based on that, but... as far as I know pathauto doesn't have any conditional abilities and I'd want to use a field_legacy_slug only if it exists, else use a standard token based pathauto pattern (site.com/2011-01-01/title-of-post)

I also don't think creating multiple alias' for content is advisable (aka letting wordpress url alias' exist beside the native pathautos) - but I am course asking you for insight into this :)

Creating node paths on import myself and allowing pathauto to catch new nodes seems like my only option but I'd hate for someone to run an "Update path alias" and ruin it all.