Perhaps because it's late in the day, or I'm having a senior moment, I'm doubting my memory.

Plan has allowed in-service distribution of amounts rolled into the plan from other plans. They would now like to amend to remove that provision. Isn't that a protected benefit as to the rollover balances in the plan prior to the date of the amendment?

Yes. But there's nothing I know which specifically names hardship distributions as protected.

Hardships are specifically exempted according to the ERISA Outline Book.

This is not related to hardships. It is an in-service distribution, and the timing of a distribution is protected under 411. I think current amounts remain/keep the in-service option, only new amounts get restricted.

Yes, K2, hardships may be eliminated (per Treas. Reg. Section 1.411(d)-4, Q&A-2(b)(2)(x)). I misled by referring to hardship distributions in my response--the OP referred to rollover distributions, not hardship distributions. (I've enrolled in a Reading Comprehension for Dummies course.)