Front Loaded Absence Plan - no changes with working hours change

We have our PTO accrual plan set up as front loaded, and the derived factors are based on length of service and FTE.

Our requirement is to update the plan so that initial balance calculations remain the same if an employee has a working hour change middle of the year. If someone goes from Fulltime to Part-time, we do not want their front loaded accrual to be modified to the lower tier. Is there a way to do this? Unfortunately, I'm not versed in fast formulas yet, so if that is the route we need to take - is there an example that I can use to meet this requirement? Thanks in advance!