回複：Need expert's advice, pjiang, 863211 please.

H1b, I-140 approved, H1b extened 3 year after 6 years based on I-140 approvel and still has 2 more years. Lost job this month and don't know if ex employer will withdraw H1b and 140 or not.

Now should I change immediately to a dependant H4 visa and change back to H1 when get a job (will I be able to change back to H1 again in this case ?)
-Yes if your I-140 sponsor/employer does not withdraw the approved I-140. If the approved I-140 is withdrawn, you can not get back to the H1 unless youu get a new I-140 approval.

or just try to find a job ASAP and transfer the H1b when get a job.
-Up to you and consider how soon and how sure you will find a job (e.g. one to two months). Otherwise, just change your status to H4 and hope your employer does not withdraw the approved I-140.