That's what I would do without further information:
I would put in your form handler a property to contain the navigation style. When you render the page, check that property (with a Switch droplet, for example), to see what kind of navigation you have to show.
In your form, do the dropdown with a dspel:select, and set the bean property to the new form handler property. In the "onchange" event in your select, call a function in javascript which submits your form using a hidden submit called "refresh" (which you should implement in your form handler), that simply refresh your page, so the new layout is displayed.
This is a quick and dirty way. You may use ajax to do all that stuff without refreshing the whole page, or even use plain javascript to avoid all the communication with the server.
hope this helps,
Gustavo.
