Your loan has already been sold on the secondary market. I dont know if you have a 10-15-30 year mortgage. You cant change the terms without refinancing.
Say you want to buy your rate down, you will have to do a new loan. You can do it with the same company you are with, but its a brand new loan.
Right now your rate seems pretty good. If you try and buy down a few points it might cost you 8,000 to save 100 bucks a month. Run your numbers very closely. In the current market your rate is good.
But no back (that I know of) allows you to just buy down your rate after you have closed. It would totally mess up your note.
I just read that again after the other answer. His answer was good and right. I assumed in my answer this was a closed loan. If you lock at 6.125 with a company. And before closing you want 5.75. Every company I know will just go back to the day you locked and give you the 5.75 and tell you what it will cost to close at 5.75 instead of 6.125. You are not set at 6.125 for your rate, you are just set to the day you locked it. You will get the buydown rate for the day you locked it, not todays. For good or bad.
30+ years mortgage/real estate exp.