Convert posts from an existing WPML multilingual site via WP Import/Export to a WPMS (Network) with Language Switcher so easily it feels like magic!
WPML stores each entry as a separate post and uses some custom tables to connect the translations. If you de-activate the plugin or have issues with functionality related to version updates, you get one site with multi-lingual confusion. See Getting WordPress to speak your language for more details how / why using a WordPress MultiSite / Network is becomming the recommended strategy for multilingual sites. At least until it is fully supported in core.
You will need to handle conversion of any strings, .po files or other language elements separately.