Person Change Causes Life Event for change of employment category

I need a FF to trigger when employment category changes from specific value to another value. There are multiple conditions.

eg: When the employee is changed from PT to FT, it triggers a life event

I have this formula. However, it doesn't detect the change

Inputs are BEN_ASG_IN_EMPLOYMENT_CATEGORY (text), BEN_ASG_IO_EMPLOYMENT_CATEGORY (text)
DEFAULT for BEN_ASG_IN_EMPLOYMENT_CATEGORY is 'XYZ'
DEFAULT for BEN_ASG_IO_EMPLOYMENT_CATEGORY is '****'

l_return = 'N'

IF ( BEN_ASG_IN_EMPLOYMENT_CATEGORY = 'Full-time regular'
or BEN_ASG_IO_EMPLOYMENT_CATEGORY= 'Part-time <24 hours')

then
(l_return = 'Y')