Well, i have a weapon with 3 skill at max level for their rank and i was trying to upgrade them to the next rank. But for some reason i cannot upgrade them, i talk to Momotarou, but it says that i don't have a weapon that i can upgrade. How does that exactly works?

Momotarou can't up skill for enchanted weapon ( ex: +4 cedar spear can't use Skill evol). In ur case, u can use weapon Smelt by Yoshitsune, view this link http://onigiri.cyberstep.com/gamesystem ... ining.html

Skill evol in momotarou can use for another weapon like:
_ U max lv wind thurst rank III with +20 cedar spear
_ now u have Ice spear with Wind thurst rank I
_ go talk with momotarou and u can up ur Ice spear Wind thurst rank I > III

Then you need to have both the rank 3 skill, and inferior ranks to get it to upgrade it or only maxing the rank that you want to ingrease is enougth?

And It has to be a weapon not upgraded, rigth?