More McDonald's McMuffin changes to come

Egg 'McMuffins' now cage-free

As McDonald's U.S. unit prepares to expand breakfast hours nationwide and transition to cage-free eggs, the chain is busy making another tweak to its popular Egg McMuffin sandwich.

It is returning to its original English muffin, according to an internal memo sent Wednesday by McDonald's USA President Mike Andres.

Andres detailed "the return of our classic Egg McMuffin featuring the return of butter in place of margarine, the original English muffin, Canadian bacon cooked in its natural juices, and, of course, a fresh-cracked egg at the center of it all."

McDonald's Egg McMuffin
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Breakfast, a strong segment in fast food, remains a key focus for McDonald's as it embarks on a prolonged turnaround attempt.

Read More McDonald's all-day breakfast is going nationwide

The memo focuses on several changes McDonald's has already implemented as it works to enhance its food, including phasing out chicken treated with antibiotics important to human medicine and serving jugs of milk from cows not treated with rbST, an artificial growth hormone.

"We're changing to meet customers' evolving taste preferences and expectations, and one change includes returning to using real butter on our McMuffins," McDonald's spokeswoman Lisa McComb wrote in an email.

The change is currently rolling out. One source said the switch at her location occurred about three weeks ago though she noted there is "not much difference" in taste.

"Right now, we haven't had any feedback," she told CNBC.

On its website, McDonald's still features an older McMuffin recipe made with liquid margarine.