Per the starship stat sheet explanation page, they become combo packs. A Module and Equipment to change between the Harbinger, the Sentinel and the Warden. All the variants have all the same hardpoints now as well. (Same pylon mounts, for example.)I do believe they will be getting changed in the near future anyways. According to Noob. I'll find the video.
Basically, BUKs will disappear, and will be turned into equipment for the ships to use as the different variants.
Edit: Here it is.
Edit 2: 4:08 - for the BLUF on the BUKs
Edit 3: 3:50 - this has been corrected: ALL the variants still have the size 5 on the nose
So based on Noobs video, and the corrections to it, basically what changed between 2.6.3 and now is nomenclature.
Note that I do hope the Sentinel continues to be Test colors, though the warden is shown as blue instead of the Green we have in our hangars.