Timing of AC21 - I-485 pending for 6+ months

[esaga]

I have a valid EAD/AP document as I my I-485 is pending for more than 6 months.  I am planning to change employers and wondering if I can submit AC21 (and now Supplement J for Form I-485 under the new rules) after joining the new employer or does it have to be prior to joining the new employer.  Thanks!