Not calculated like that.

Loan interest is calculated daily.
if you have outstanding loan let say 250k, it will x the new BLR (if) with daily calculation.

There are 2 situation can occur,
1. Monthly loan repayment increases based on calculation of the whole tenure of the loan vs its interest rate.
Aka previous if BLR 6.6% enable to repay off the 25 year loan with 1000 monthly, with the new increased rate, it become 10xx.

2. Monthly repayment still stay the same, 1000, but to finish off the loan take more than 25 years.

My view, unlikely BNM want to move the rate prior before GST implementation.

It may be one off 0.25% increase then no move until the GST implemented, as there are plenty of uncertainty factors
with GST implementation with potential one off inflation due to GST, and economy slowdown due to GST as well.