So I just got a riven for something I use for the first time.

It's a twin Grakata (YES TWOOO) riven. +Zoom -Recoil

It's MR16 so I can't use it yet (soon) but how does rerolling with Kuva work exactly? Do I reroll one stat or the whole thing? (All stats, new weapon, new mr?)

If I do so, can I still sell it? If I can, can someone reroll it after I do?