Talk:Items: Skill Orbs/@comment-36323615-20200202120157/@comment-32271305-20200202121040

The upgrade is entirely optional. If you get a Skill Orb that increases the stat by 300, it's your choice whether to use it or not, and on which character. You can also replace it with a different Skill Orb if you find a better one.

As shown on this page, different Skill Orbs also increase the stats by different amounts, so your last question can have many different answers.