Okay, Tahra's question got me all intrigued, and since I have plenty of babies to spare, I thought I'd try this one for myself.
There's some good news, and some bad news
1) It still is relatively easy to just pull out the heads of the oldest style babies. Here's an example of one I popped just now:
2) With some effort, I also managed to yank out the head of a newer style baby. I used one with the regular Playmobil haircut rather than the nice wavy one, because I like the wavy hair better. I pulled it, and I think that loosened it a little, and then I thought: "Let's just use Tahra's fishing line method and see what happens!" It worked
Then I tried to switch the heads, but that is not possible. The oldest style baby's head does not fit the narrower hole for the newer style body: the protruding bit is too wide. You might even see it on this picture:
3) I then decided it was time to pop the newest style baby's head. The fishing line method seemed to be working, so why not? Unfortunately, they changed the design on the head again. What happened is, they've secured the head of babies even more. The protruding bit has a wider part at the end, so it is firmly stuck on the body. If you really, really try to pull the head off... it just snaps
4) The good news is, that I don't like the newest style heads either, so I'm not too bummed out about this mishap, AND, what is more, the heads of the newer and the newest style babies ARE interchangeable
So this little guy now has fully operating hands and a slightly less ugly head!
This means that it is possible to change the heads, but then you'd have to sacrifice the newest style heads. With their lovely hair colour varieties.
Hope this answers your questions, Tahra