One Interesting question.

Hello Guys.
I have costumised Dragos Flatbow with 10/10 armour penetration.
Can Arena net upgrade to 20/20?

They 'can' change the mods of weapons wherever they are and whether they are customised or not. But they are unlikely to do it.

they can do it doesn't mean they would do it. Bill gate can give you 10M but he wouldn't do it. And anet will not change the mod for you.

Unconfirmed

Read in a Gaile log that when the armor upgrade NPC joins the game u can upgrade your existing armor to the new specs without cost and also upgarde 'changed' greens to their new specs.

where can i find this npc?

You cant. Yet.