How can I convert between different datums?

You have several options...

Here is a web site that will do it for you: http://jeeep.com/details/coord/

You can find software to do it using your computer. The best solution if you have lots of points to convert.

You can use you GPS unit:
Store the coordinates as a waypoint or landmark, switch the GPS to the desired coordinate system/datum, and recall the stored position.