When you lock a rate, your lender is making a commitment to deliver your loan (if it closes) into a certain investment pool, which is where your rate came from in the first place. Pricing for a rate depends on the length of time the investor is expected to "hold" your rate for you... lock periods are usually 30 days, but can be a short as 10 days and as long as 360 days.
nce a rate is locked, the investor charges a fee to extend since loger rate lock periods cost more. If the rate lock expires, the investor applies "worst case pricing" which means any discounts avaliable when your rate was locked will not be applied to re-lock.
As a rule, investors refuse to allow lenders to request a new lock until 90 days have passed. This is to prevent borrowers from rate-jumping when rates go down, in the same way borrowers are protected from rate hikes on locks when rates go up.
If you wish to re-lock a lower rate, you have three options:
1.) Pay the lender a fee to re-lock - there is no fee if your lock provides for a "float-down" option
2.) Withdraw your application and apply with a different lender
3.) Withdraw your application and wait 90 days to re-apply.
Bear in mind that a rate lock works both ways: Your rate doesn't go up (during the lock period) in a rising rate environment, and your rate doesn't go down (during the lock period) in a falling rate environment. The purpose of a lock is to do just that: lock.