Post I140 Rules

Hello Experts,

If someone has I-140 approval notice, post then does he needs to be with the same company for 6 months before he wants to switch to another company to port out his priority year / month? does this 6 month of timeframe documented anywhere in USCIS website mentioning this as a valid rule? Curious to know more on this.

Secondly, if a company files I140 in EB2 category, and if he switches to another company and if he files EB3, in that case which one will be the track for the person (EB-2 or EB-3) ? Can he be able to transport his EB2 priority year/month to the new employer?