horrgakx, Hi
Replacing the handles is a relatively easy and straight forward procedure.
The existing handles will come of if you simply unscrew all 4 holes on each handle one any one door, you will find that there is a square bar that passes from handle to handle and obviously actuates the latch mechanism.
Your problem is that should you opt to fit a set of handles that are a different size to the metal plates actually attaching to the door, not the handles, but the oblong metal parts with the 4 Screws. If you try to fit a different shape of this component, the oblong metal part then, should your new handles be of a lesser dimensions, or indeed a circular plate then two potential problems arise.
1/. If the fixing plate with the screws is smaller than these fitted then the face of the door will have two potential problems, one, the screw holes from the existing plates will be visible, and difficult to obscure totally.
2/. Over the time that the existing plates with the screws have been fitted to the door, the plates will have in effect etched themselves on to the surface of the door, the constant usage of the handle and its associated tiny movements of the plate can cause the appearance of a, best described as a Ghost of where the original plate was fitted. Plus the colour on the door below the original plate will be different to the rest of the door, not having been exposed to light.
One wee bit of advice? if you do undertake a DIY job, then at all times put the square metal bar and at least one handle in your pocket, why? because if you inadvertently lock yourself in a room, the door slams closed with you in the room? then it is a bit of an embarrassment, but a flat bladed screwdriver will get you out of the room, how? a correctly dimensioned flat blade driver acts in the same way as the square bar??
Hope this assists??
Ken.