can employer change SOC code on I-129?

Generally speaking, the SOC code on the H1B registration, LCA, and I-129 petition should be consistent. USCIS adjudicators crosscheck the SOC code on all three applications, and inconsistency would invite a RFE and likely denial. In your case, with the initial SOC of 43-6012 being legal secretaries and administrative assistants and SOC 23-2011 being paralegals and legal assistants, it is apparent that the latter SOC code is better suited to fit the H-1B requirement of the position being in a specialized occupation. There is also the issue that the wage for SOC 43-6012 is less for legal secretaries and administrative assistants than for paralegals and legal assistants and that the organization may have used that fact to get a higher level wage thereby increasing the chances of the H-1B registration being selected. Perhaps the best solution is to continue using the SOC code and explain exactly how the job in question qualifies as a specialty occupation requiring at least a specialized baccalaureate degree.