ginnu,
i agree with what you say, however, as you know, AC21 states the validity of approved I140 when jobs are changed and NOT that I140 approval is required to change jobs.
------ you CAN change jobs without I-140 approval, you can join other employer on H1, but to USE AC21 I-140 APPROVED and I-485 pending more than 180 days is needed, if you dont want to use AC21 or dont care about AC21, you can change employer any time
in cases, where applicants are assured support by their sponsoring employers on not withdrawing I140 if the beneficiary moves to a new job, what do you think, can they take the new job?
----- if the past employer does NOT withdraw the I-140 or if RFE for I-140 and replies the RFE and RFE responce is accepted by USCIS and then I-140 is approved AND I-485 is pending more than 180 days AND you find Similar job and your New employer is ready to give you Future permanent job offer letter