I'll start by quoting the eminently quotable keithy, from 9 June 2015 (Subject: Garmin Etrex 20 GPS Help):
Familiarise yourself with the different Position Formats (make sure you are using the correct format and datum corresponding with your paper maps) so that when you enter a waypoint manually, you are actually entering the intended position. Using the wrong datum, for example, will result in an incorrect location.
As it happens, I'm using that Garmin model, but my question relates to any GPS device.
I am creating waypoints from an old AGD66 1:25k paper Vicmap, to enter into my device pre-loaded with andrewp's latest Vicmap and OSM maps. (Thanks once more to the equally eminent andrewp.) I assume the latter maps are GDA94. Prior to my walk, I am converting the AGD66 coordinates to GDA94 coordinates. When I am walking, I'll be combining these converted coordinates with a recently downloaded GPX file of a route in the same locality. Therefore I will obviously have the Garmin position formats set for the entire trip as UTM and GDA.
Is the way I'm planning this correct? Another way to proceed would seem to be to reset the position format to AGD66 for one part of the walk, then change to GDA for another section. Unless I'm misunderstanding something, that way I'd avoid having to do the conversions at home before going out. Which procedure is better?
Note, I'm far from proficient with GPS theory, so a lot of the info I've read online about this issue is more or less meaningless for me.