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News says it is a 747-400, but it doesn't have a winglet.

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  • $\begingroup$ Good question. The same aircraft with JAL in 2010 has the winglets. They’re still there while it was stored at KVCV in Feb. 2011 before it was sold to GE. There’s an article here describing modifications made to the aircraft to accommodate testing. It doesn’t mention winglets but it does mention modifying the wing structure $\endgroup$ – TomMcW Mar 24 '18 at 17:32
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That's because its been removed for testing. The aircraft used in for flight testing the GE-9X belongs to GE and is a modified JAL 747-400.

The aircraft, a former Japan Airlines -400, was acquired by GE in 2010 and following maintenance and interior modification in Xiamen, China, by Taikoo Aircraft Engineering, was delivered to Evergreen Aviation Technologies in Taiwan for installation of data racks and instrumentation.

For carrying out the testing, GE has carried out a number of modifications for maintenance and testing:

... modifications are also planned to reduce the chances of aero elastic interaction between the extended wing of the 747-400 and the GE9X mounted on the inboard left strut. GE has already removed the standard winglets because these add unnecessary weight and a maintenance burden to the aircraft’s flight-test role. Later this year, the outer 6 feet of each wingtip also will be removed, ... Designers say the stiffer wing will reduce potential exposure to limit cycle oscillation and maintain an unrestricted test envelope.

Quotes from archive.org. Original ariticle appears to be from aviationweek.

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  • $\begingroup$ The winglets were also removed from the 747-400 Dreamlifter due to vibrations encountered during flight. citation $\endgroup$ – TomMcW Mar 25 '18 at 1:30
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To quote from this article in Popular Mechanics,

In preparation for the flight, GE removed six-foot wing extensions from the 747 to reduce the wingspan to just over 195 feet, stiffening the wing to support the weight of the giant engine.

i.e. the 9X engine is heavier than the CF6 that is was replacing. Therefore the wing needs to be stiffer. If the wing was not stiffer, then the natural frequency would drop due to the heavier engine, and there may be a wing flutter concern. The wing is basically a big beam, and shorter beams are stiffer than longer beams.

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