The airline’s decision affects Boeing’s 737 jet, one of its biggest moneymakers, and raised some fears that Boeing may not lift 737 production as planned and could miss financial targets. The world’s biggest plane maker said United’s move would have no impact on production.

United (UAL), the No. 3 U.S. airline by passengers, said it will convert orders for 61 Boeing 737-700 planes due to be delivered in the next two years into the newer 737 MAX model, but that it was still deciding on which size MAX planes to order and when they would be delivered, creating uncertainty for Boeing investors.

United also will convert orders for four 737-700s to larger 737-800s due for delivery in 2017.

Boeing shares (BA) fell 1.8% in early trading on the New York Stock Exchange and were last down 1.6% at $147.56. United shares rose 3.4% to $65.08.

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“We don’t know when are they (the MAXs) going to come but they are going to come as 737 MAX, most likely MAX 8 or 9s,” United Chief Financial Officer Andrew Levy said on a conference call with analysts. “The reason we are doing this primarily is because we just don’t need these airplanes right now.”

Boeing Sticks to Plan

Boeing said United’s move would not alter its planned increases in 737 production rates and stressed it continues to have orders for more 737s than it can produce. Boeing will keep the 737-700s on its books until United finalizes the MAX order.

United’s orders “will switch over to MAXs on the order book when (the order is) finalized,” Boeing spokesman Doug Alder said. Boeing’s 737 order backlog of 4,321 airplanes “gives us the flexibility to meet those needs.”

Boeing affirmed recent comments by Chief Executive Dennis Muilenburg that 737 output will rise to 57 a month in 2019 from 42 a month currently with interim steps to 47 and 52 a month.

“Importantly, even at the 57 per month rate, we continue to be oversold,” Muilenburg had said on Oct. 26.

Some analysts said United’s decision would only reduce the number of oversold jets, with no impact on output.

But others worried that converting 61 current-generation orders to MAX orders implied a production delay. Boeing’s factory is phasing out current generation 737s as it switches to the MAX, which is due to enter service next year.

“It looks like 737 output will not grow as planned,” said Richard Aboulafia, an aerospace analyst at the Teal Group in Virginia. Boeing is already cutting production of the 777, its other cash cow, and 787 output is due to remain steady.

For more, read: Boeing Is Cutting 500 Jobs From Its Defense and Space Units

Hitting 52 a month “seems to be a slam dunk,” Russ Solomon, analyst at Moody’s Investors Service, said of Boeing’s rate increases. But reaching 57 a month “remains more of a question mark.”

He said Boeing is supposed to begin generating more cash from 787 production starting next year. Moody’s was not altering its rating on Boeing.

“They actually have a bit of a ‘rich’ problem” on 737s, Solomon said. “They probably cannot deliver all of the planes they have committed to near term.”