No, I tried it. I put item +12 to vault and go to web and item don't appear to be able to upgrade.
In "Rankings, TDM, Set bonus, +14/15 uplevel price" topic, Arthur said about price from +12 to +13 too but it doesn't work in web.
In server that I used to play 10 years ago, the calculating system is like that, the more lvl you reset more point you get. So I think this server might also calculate like that too. So there is no such calculating system in this server.