Website Update Data Migration Advice Needed

I currently run a old website that has good traffic with over 250,000 indexed pages but it is over due a update/overhaul

I am going to update to a new website design entirely and transferring all data, pages, mysql, users but i will encounter many urls not found, de-indexing and then re-indexing, serp penelties etc

Is there a good way in which to migrate data for this type of update.

I appreciate all of your help

 

Create an xml map of your site first.

 

Yes create the xml file and use either a scraper or file editor in bulk to assemble your site again.