I am on my 6th year of H1b visa , I have LC and I 140 approved with my current employer.Cannot file I485 due to retrogression of dates. I want to switch jobs now. I ubnderstand that my new employer has to re-do everything starting from LC .My questions are
1) Can I keep my original priority date ? Is it a fairlysimple process or USCIS gives a hard time porting the Priority date?
2)If my PD beomes current in the next few months after I leave my old job before my new employer files new labor, can I file for I 485 if both the jobs are of absolutely similar category /education level etc?
3) How long can I keep extending my H1b based on approved LC and I 140 ? Is there any upper limit or I can keep extending even after 10 years if the dates are not current?

Thank you for your help