The biggest issue will be in the need for proper 301 redirection so errors are not found. There are plugins via wordpress to help you along with that. As long as wordpress properly redirects old post locations to the site.com/blog location you should have many issues or loss of rank etc.
I would recommend contacting your host(hopefully not being godaddy) to help you in the transition/swap to make sure nothing is lost. as some server side issues need to be changed to cause the least amount of hiccups.
Pay attention in your webmaster tools after the move to make sure you're on top of any issues.
Lastly, you are not changing domains, but are changing the permalink structure of your site.
