Change a mixdown resolution or relink it to 4k

I started a 3840 x 2160 project and imported at 3840 x 2160, however when I mixdown the files they came out as 1440x 1080. I already cut the mixdown clips and I hate to have to remixdown the imports and recut the clips in order to get frame flex. 

I did mistakenly bring in the clips as UHD 30p project when the footage was shot 29.97fps.

I did notice a mixdown of the same clips just below was 3840x2160 however that sequence will not allow frame flex also.  

What is keeping me for getting frame flex and can I change the 1440x1080 mixdown?

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