If u change ur job now, ur sponsoring company can revoke your i-140 which would invalidate ur GC application,EAD,AP and as you know that once u use EAD,AP u loose h1b status, bottom line is that if u change ur job before i-140 is approved, and ur i-140 is revoked, u would be in real fix, if u think ur sponsor would not revoke ur i-140 app, then u may consider taking this offer, if u have even 1% doubt that he may revoke it, then U MUST WAIT FOR I-140 APPROVAL.
just a friendly advice
-thx