Hi,
The Personal Savings allowance is based on which income tax band you are in, this is calcuated based on your total taxaable earnings in a tax year i.e 6th April 2024 - 5th April 2025, so becoming a higher rate taxpayer mid tax year does not autaomatically mean it will impact you Savings allowance.
Please see
Tax on savings interest for further information on the different bands.
Thank you.