The Etrex can use them from the SD card – you don’t need to transfer them.
Copy the gpx files to the \Garmin\GPX directory on the SD card(s) (or drag and drop to the card from Basecamp, etc.).
The track names should all be visible in the ‘Track Manager’ on the Etrex (press menu button twice and click on track manager then scroll down to the track you want). You need to go into each one and click ‘Show on Map’ (which turns the button to ‘Hide on Map’).
Bear in mind that the “Tracks” are just overlays on the map to highlight your route (as distinct from a Garmin “Route”) so that whenever a relevant part of the map is in view, the Tracks should show up, provided they are set to show as above.
(FWIW: I would go for a backup that didn’t involve the Etrex. The subset of problems that could be fixed with a spare SD card is probably very small /FWIW.)