You are obligated to inform the lender of any change in circumstances that affect your finances.  This certainly will affect a pre-approval.  The lender will find out during the verification process for your application, anyway. 

If you do find new employment, and it is not totally unrelated to your previous employment, you still may end up qualifying for a mortgage.  The same holds true for any major change in financial circumstances.  Buying a new car, for instance, is best left until after the housing transaction is complete uless it is absoultely necessary.  If you have a copy of your application [form 1003], check the verbiage above your signature on page 3.  Honesty is always the best policy.  And today more than ever.