Short answer, yes.
There are a lot of life changes that will make it a necessary task to change a W-4. Getting married or divorced, having a child or other changes in dependents will alter withholding allowances. They may want to add dependents to decrease the amount withheld, but in general it’s ok for an employee to change withholding allowances at any time and for any reason.
When a new W-4 is turned in, the new entry should be in effect no later than the start of the first payroll period ending on or after the 30th day from when it was submitted. This will replace any previous W-4 amounts the employee had before.