You will need to email Garmin Australia and tell them how disappointed you are with their latest software update for the Nuvi 500 as far as geocaching is concerned Then ask them for a previous firmware (I think it is 3.90) where the pedestrian arrow still works.
MissyLJ did this and they sent her an old firmware for her device, also her gps is still covered under warranty, this may have had a bearing on actually getting the old firmware.
I have backed up my own Nuvi every time before I did an upgrade.
The only other way is if you have backups of your unit before you did the upgrade to 4.20 is copy your backed up GUPDATE.GCD back to your device then reboot it. It should ask you about going to a previous version. Accept this and all should be good.
************* BIG DISCLAIMER HERE ************************
*PLEASE NOTE* If you follow my instructions here and screw up your gps I will not accept any responsability for it. It is entirely your choice and risk to do this.