Replacing Runes ??

I was wondering. If you have a rune already in a piece of armor, can you replace it? Say I have a Minor Rune in my leggings and I found a Major Rune I want to use instead. Can I just double-click and put the Major Rune in the leggings? Or once a rune space is used, is that it?

I know they don't stack, I'm just wondering if it is upgradable.

Yup. Works the same way for weapons as it does for runes -- double click the upgrade and it will be applied over top of whatever is already on the item. I'm pretty sure there's a confirmation dialogue before it becomes official, but it's been a while since I did this, so I could be thinking about something else...

Cool, thanks!

Do you lose the Rune that was in the armour previously?

Yes the previous rune is lost.