Depending on the date of manufacture it may already have firmware 2.0, assuming the later firmware is now used in production. The easiest way to check is using a midi interface and the Line6 monkey.
If you're happy with the 4 standard voicings then no there's probably no reason to upgrade the firmware. That said I'm fairly certain you can already access the additional voicings using your HD500, the main purpose of the firmware update as I understand it is to allow access to the voicings through midi cc.
To use Line6 monkey and view/update the firmware version you need a midi interface. The simplest solution is a USB midi interface, you can get them for ~$30-50. Ideally get one that has the midi cables attached, one end will be USB and the other will be two 5-pin DIN connectors which connect to the back of the DT-25, see below image
That particular model is the "M-Audio USB Uno"
hope that helps