if (cnt > 50, avg_account, else null) as avg_account
if (cnt > 50, avg_change, else null) as avg_change

select      id, branch, account, total, change,
            'avg_account' = CASE
            WHEN cnt > 50 THEN avg_account -- How can I refer  to avg_account values?
            ELSE 'Null'
            END,
            'avg_change' = CASE
            WHEN cnt > 50 THEN avg_change  -- How can I refer  to avg_change values?
            ELSE 'Null'
            END
into       Table2

from       Table1
           print '  ' + convert(varchar,@@rowcount)+ ' rows updated'