Season 12 is 483 and Season 13 is 496/512. Even fully upgraded, the Season 12 gear isn't as high in level as the Season 13 gear.
One reason Season 13 won't be upgradable is because PvP item level is being capped at 496, which is what the Season 13 gear already is. The other is that the upgrade system worked counter to the main form of gearing in PvP (i.e. they both used the same currency, and there's no boss-drop style of gearing like there is in PvE), so it created painful decisions that the developers didn't like.
I could care less about itemlevel, I mean the stats on the gear.
Season 12 elite is better than season 13 conquest, Why even make a new season if old teir is still better stat wise?
Er...it doesn't? I'm comparing the items and the Season 13 items have higher primary stats, higher secondary stats, and higher PvP stats than the Season 12 fully-upgraded items. Unless there's something wonky about them that's not reflected in databases, the Season 13 gear should be superior in every way.
Now, the Season 13 Honor gear does have slightly higher PvP stats than the Conquest gear, but that's it.