Legacy mode not supported on pages that use UpdatePanels, but...
you can still use it at your own risk.
By default, if you set the conformance level to debug, you'll get full postbacks in your pages. why? well, because during the Init event of the page, the PageRequestManager server class initializes the SupportsPartialRendering property of its owner (whic, btw, is the ScriptManager control) by checking the value of the EnableLegacyRendering (which icomes from the property with the same name that is exposed by the ScriptManager control).
What you might not know is that if you set the SupportsPartialRendering property the previous execution logic never takes place and you'll get partial postbacks working at your own risk (might I add:) )