The newer masters for the DVDs were carefully IVTCed (and the 29.97 portions brought to 23.976 in I-don’t -know-what-way), and then sped up to 25fps.
Are you sure they did this?
From what I found it was suggested that they used a method called DEFT to convert it to 25 fps. It does produce some combing artifacts but it works 99% of the time and that’s why I haven’t touched the PAL discs since then.
The portions with originally film, 23.976fps, seem clean progressive, and just sped up. That’s why the German sound for the DVDs had to be sped up too, while in the nineties they had done another type of conversion without speeding it up. Often done at that time, but not suitable for modern Flat tvs. This was a mixture of keeping the original interlace structure plus blendings - sometimes quite well reversable with avisynths SRestore.
The portions with originally 29.97fps (cgi, and, as Joel pointed out, even more scenes) have to be converted in another way: To get them from nearly 30 to 25 it is not possible just to slow down, that would be much too highly noticable, for sound and also picture. Also decimating is not possible, because there are no double fields to remove, and the result will be very jerky. So this is done with motion flow algorithms, f. e. with hardware or software alchemist (which can also DEFT, as I just read, but I don’t know anything more about this special technique).
In this special case they had to bring it first to 23.976fps, so that the whole episode in the end could be sped up alltogether to 25fps for PAL.
This is all a bit speculation, because up to now I haven’t seen any of the NTSC DVDs to be sure, but it should have been made like this - I can’t think of any other reason, why the German sound was sped up for the DVDs. And this was used typically.
So you can simply take the PAL discs, slow it down to 23.976, and you have the best possible conversion already done to
-one unique framerate
-progressive (if the original 29.97fps sections ARE progressive, I just assume this - if not, deinterlace first)
Everything you need for upscaling.