So that the rate changes from 4.625% to 2.75% and the monthly payment can be reduced roughly by $2000
following are the data I made some adjustment
30 年 利率 4.625%
100万 月$5141.39
80 万 月付 $4113
78万 月付 $4010
28年利率2.75%
58万月付 $2477 50万月付 $2135 48万月付 $2050
60万月付 $2562