I485 portable means that your job description is similar between your previous company and new company, it depends on the description on your old LC application and your new company's job description for that position.
That is why I think it imply both of company positions are good for GC applicant, i.e. the position can not find US worker to fit for it, and willing to hire a foreigner to work for that and company is willing to sponsor your GC.
Though it is true that practically, most of people get I485 approved without issue, even they do something violate the rule or doing something in grey area. But I just want to remind you that there could be potential risk for you. If USCIS need something from your company for I485 REF, you will not able to get it, since your new company does not want to sponsor your GC.
I think you need prepare for that worst case.
That is why I think it imply both of company positions are good for GC applicant, i.e. the position can not find US worker to fit for it, and willing to hire a foreigner to work for that and company is willing to sponsor your GC.
Though it is true that practically, most of people get I485 approved without issue, even they do something violate the rule or doing something in grey area. But I just want to remind you that there could be potential risk for you. If USCIS need something from your company for I485 REF, you will not able to get it, since your new company does not want to sponsor your GC.
I think you need prepare for that worst case.