Bhargavi,
In general, you should not change your lot size in your material masters unless there is a compelling business reason to do so.
Just the act of running MRP more often than once per month, by itself, is not a compelling reason to change the lot size. MRP is quite happy to run using Monthly lot size, as often, or as seldom, as you wish.
Instead, you have to ask yourself what are the reasons for the new MRP schedule, and what are the business users' expectations from the resulting MRP run. You can easily see the results by experimenting in your quality system.
Best Regards,
DB49