EB2, 140 approved, 485 filed in July 2007, changed job in July 2008, can I change job again as long as it is a similar position?
-Yes.
When I changed job in July 2008, the company's lawyer wrote a letter about AC-21.
I am currently using H-1B (the 6th year), and have EAD in hand, Is it safer to do H-1B transfer or just use EAD if I change job the second time?
-Up to you. Either way is OK.
Do I need to hire a lawyer to do the AC-21 again?
-Not need. If you get RFE for your employment status, you just ask your employer to provide a letter. Absolutely you do not need to give your money to a lawyer to deal with such a simple thing.