SQL Z97 if INSUB and Greater than 199

How can I get matlxfer.xtype to equal 'Z97' if matlxfer.xtype = 'INVSUB' and matlxfer.xfer_vndlot > '199' and '551' if it's less.

 SELECT matlxfer.fm_cost_ctr ,
        matlxfer.to_cost_ctr ,
               CASE matlxfer.xfer_vndlot
        WHEN '200' THEN '1200'
        WHEN '201' THEN '1201'
        WHEN '202' THEN '1202'
        WHEN '203' THEN '1203'
        WHEN '204' THEN '1204'
        WHEN '205' THEN '1205'
        WHEN '206' THEN '1206'
        WHEN '207' THEN '1207'
      ELSE matlxfer.xfer_vndlot
      END AS xfer_vndlot   ,
        CASE matlxfer.xtype
          WHEN 'ADDINV' THEN '552'
          WHEN 'INVSUB' THEN '551'

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xfer_vndlot	ADJUSTMENT
108	551
109	551
110	551
1200	551
1201	551
1202	551
1203	551
1204	551
1205	551
1206	551
1207	551

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