They are most likely gonna do what they have done with the overworld in previous updates and leave anything that is built/generated alone and any new chunks that get generated will be able to spawn the new stuff. It is possible to see this with old 1.0-1.4 worlds as the world generator was changed in 1.5 and newer. this causes oddity's like ground level variances that i've seen in the worlds i've moved forwards! XD the picture shows my old pre v1 minecraft world. (i've got several old snapshot worlds that are still working as well)
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