Amend FSA that Utilizes Grace Period to Carryover

Employer maintains an FSA plan that provides for a grace period.  Calendar year plan.  Regs state that if it has a grace period it cannot also provide for a carryover.  The Regs state that it can be amended prior to end of year to change.  So, under the Regs, a calendar year plan permitting a grace period in 2026 relating to 2025 could be amended to instead use a carryover to 2026 of unused 2025 health FSA amounts (as limited) if amended by December 31, 2025.  I didn't think you could do it this late but the Regs state differently.  However, Notice 2013-71 states "If a plan has provided for a grace period and is being amended to add a carryover provision, the plan must also be amended to eliminate the grace period provision by no later than the end of the plan year from which amounts may be carried over. The ability to eliminate a grace period provision previously adopted for the plan year in which the amendment is adopted may be subject to non-Code legal constraints."  

Can someone expand on what "subject to non-Code legal constraints" means?  I have some thoughts but would like to hear from others.

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.

Thanks, Brian