None of the 90s BBC Two idents (nor those added in the early 2000s) were created in HD. The fact that Network is not using certain idents is possibly more to do with the fact that they don't have clean, broadcast quality copies of those idents in their archive.
If the original film still exists, it's possible that it could be rescanned into high definition, like the high definition transfers we've seen on various TV shows that were shot on film like Inspector Morse. It is doubtful that network would go to the trouble though.
NI appear to be showing idents like Balloon which were never transferred to widescreen because they were dropped with the 1997 refresh. They have presumably made their own widescreen versions by doing the same crop-and-stretch job that was done back in 1997, as NI always seem to go the extra mile when it comes to their presentation.