Generally speaking, I think you'll find that if you stick to the same number of heating zones, a new hob will fit an existing worktop aperture.
The reason I say this is that I used to work for an electricals retailer and all the different 4 burner gas & electric hobs dropped into standard pre-cut apertures in the display stand and were interchangeable with one another; they just had differing widths of lip on the worktop side of the hob.
There are always going to be exceptions though, so if you don't want to do any modifications to the worktop it's worth measuring the existing aperture (which will mean you'll have to remove the existing hob or consult it's installation instructions) before you go out shopping for a replacement.