Here are the steps to create a loan amortization schedule for variable interest rates using Microsoft Excel:

1. Open a new Excel spreadsheet and create the following headers in the first row: Payment Number, Payment Date, Beginning Balance, Payment, Interest, Principal, and Ending Balance.
2. In the second row, enter the initial loan amount of 2500000 in the Beginning Balance column.
3. In the Payment Number column, enter the numbers 1 through 156 (13 years x 12 payments per year).
4. In the Payment Date column, enter the dates of each payment. You can use the Excel formula "=DATE(YEAR(start_date),MONTH(start_date)+n,1) " to calculate the payment date for each month, where "start_date" is the date of the first payment and "n" is the payment number minus one.
5. In the Payment column, enter the formula "=PMT(rate/12,term*12,amount)" to calculate the fixed payment amount for each month, where "rate" is the annual interest rate, "term" is the loan period in years, and "amount" is the loan amount.
6. In the Interest column, enter the formula "=balance*rate/12" to calculate the interest portion of each payment, where "balance" is the beginning balance of the loan for that payment.
7. In the Principal column, enter the formula "=payment-interest" to calculate the principal portion of each payment.
8. In the Ending Balance column, enter the formula "=balance-principal" to calculate the ending balance of the loan for that payment.
9. For the first 12 payments, use the interest rate of 8% in the Interest formula. For the remaining payments, use the interest rate of 10%.
10. Copy the formulas down to all 156 rows to complete the amortization schedule.

That's it! You now have a loan amortization schedule that takes into account variable interest rates. You can customize the formatting and layout of the spreadsheet as desired.