My bathroom faucets and tub fixtures are still in good working order, I'd just like to update the look. How do I know if the trim will fit my existing valves, etc?

That is a very good question!  Valves can be very specific to the manufacturer, even the model.  I would recommend contacting the manufacturer of the current fixtures and inquire for any trim kits that would work with the existing valve.  Then let us know so we can check what finishes come with that model and availability.