Maintaining URLs already indexed when converting site to MODx

I am planning to replace a current B2B software site using MODx. Business is dependent on native search on Google etc for lead generation and has many well indexed URLs.

One reason for moving to MODx is the ability to have SEO friendly URLs which means that the new site will have different URLs for the same/similar content

Would appreciate some advice on options for implementing a replacement site with MODx and how MODx SEO gurus manage site replacements while maintaing search engine linked URLs.

Use one of the multiple alias plugins available. This will allow you to have redirects from old URLs to the new ones. The most popular plugin for this is VirtualAliases: http://www.stanback.net/code/modx/virtual-aliases.html