Old Ikea Pax corpus and new sliding doors?

Hello everyone, I hope you can answer my question. I don't really trust Ikea on this. We have 4 Pax carcasses, or carcass units! Dimensions 50 100 100 50. So in total 3m and 236cm high. Now we want to get new fronts. Because the carcass is still absolutely intact. We are thinking of fronts in 4x 75cm AULI/ ILSENG for example. Because we would like to have 2 mirror fronts in the middle. Now Ikea says that I can only install these fronts with 75cm carcasses. I don't understand that, because in the end it is still 3M. I would rather not buy new Pax carcasses for a lot of money, although ours are still in great shape. Can you help me and tell me if that is possible or if not, why!? Thank you very much los Wochos

Hello,
the rails at the bottom then do not have cutouts in the right places. Depending on your willingness to tinker, you will be able to mount the doors on the old [Korpusse].
In the assembly instructions you can see the cutout at the position of the side walls.

Hello IKEA expert,
Thank you very much for your quick response. I have no problem with a willingness to do crafts. But I still don't quite understand.
What do I have to do when attaching the guide rails? And what should I pay attention to? Is it complicated?
Best regards
los Wochos

Take a look at the lower rail in step 5.

You can see there, where the two carcasses meet, a recess in the rail so that it can be mounted. For this to work with your assembly, the recess must be in a different place, or you have to notch a groove on the side walls so that the rail can be inserted there.
In the end, you can only try it, and if it doesn’t work at all, use the 75 carcasses. However, it might be more expensive because then you might not be able to benefit from [los pax wochos]. I don’t know the exact conditions for the €40/m discount.

Thank you very much. Now I understand. I can manage that. And thus it's cheaper... despite the Ikea-Family Pax promotion weeks

Los wochos

Of course without guarantee that there is not another problem. That no drawers can be used in the 100 element is clear.