Silverlight is in some ways a "compatible subset" of WPF, but in other ways it is not. You will most likely have to modify your XAML and backing code if they are of any complexity. The way you setup and run animations are different, data templates differ, many functions and properties available in WPF classes are not available in the corresponding Silverlight classes, etc.
I ported my Silverlight app (a game) to WPF and it took about 3 hours of fiddling. Going the other way (WPF to Silverlight) can be much more difficult because you may be using a feature of WPF that has no direct Silverlight equivalent and have to do a workaround.
I don't know the specifics about differences in triggers and the visual state manager availability. It would be great for Microsoft to publish a comprehensive diff of WPF classes/functions/properties and Silverlight. Right now the only way I know is to search MSDN for the WPF class then do the same with the Silverlight version and compare what is available, or use intellisense in the code!