You can either buy them from another player via the Grand Exchange. Or you can do the barrows minigame south of Canfis. I suggest you look at a minigame guide before you attempt to do so.
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No,it is not possible to change platelegs into plateskirts unless you trade with another player.
You can either buy it or challenge the barrow brothers and if your lucky you may get dharoks great axe.
Probably Platelegs, but I'm not 100% sure.
Bandos tassets, Dragon Platelegs, or Veracs skirt
Guthix legs, they have better defence in everything but ranged.
world 2 is the best world to buy everythings
To wear Dharok's equipment, you need 70 defence; for the greataxe, you need 70 attack and 70 strength.
If you had 99 Constitution, and you also had lowest LP, you would be able to hit a 589.
Dharoks and Veracs
You are able to get dragon platelegs in 2007 runescape, but a platebody does not exist in that version because it had not been released yet. The dragon chainbody is obtainable though.
The easiest way to get it is to buy it from the Grand Exchange. There is also a plateleg shop from which you can buy it, but it's actually more expensive there. Yet another way to get adamant platelegs is to smith them yourself, from adamant bars - but this, too, is more expensive (due to the price of the bars) than actually buying the platelegs on the Grand Exchange.