Here's my take--
https://www.newfront.com/blog/the-550-carryover-vs-the-grace-period
Important Note for Health FSAs Moving from the Grace Period to the Carryover:
Employers generally should not amend a plan that offers the grace period mid-year to convert to the carryover for the current plan year. Employees may have made their elections intending to utilize their health FSA balance during the grace period by combining a year-one and year-two election for a high-cost procedure (e.g., laser eye surgery). IRS guidance suggests that this approach may be subject to non-Code legal restraints, such as an ERISA breach of fiduciary duty claim. Any such amendment to move from the grace period to the carryover should be made in a manner that ensures employees are aware of the change when making their health FSA elections at open enrollment.