You you can switch from a normal post to a recipe post with a plugin like Post Type Switcher.
Important: make sure the permalink structure for your recipes is the same as the one for your normal posts. By default WP Ultimate Recipe uses the /recipe/ slug, but if this doesn’t correspond with your current setup your old links won’t work anymore. Potential solutions:
– Change the /recipe/ slug on the Recipes > Settings > Recipe Archive page to /your-current-slug/
– Remove the /recipe/ slug on the Recipes > Settings > Advanced page
– Use the Custom Post Type Permalinks plugin for more complex permalinks like /year/month/