H E is right but it's probably more an electric issue than a water issue as if the pipes are in the wall surely you adjust the position of the shower unit accordingly and retile as necessary rather than repipe.
For an electric point of view no you can't use ANY shower. If its currently an 8.5 kW unit and cabled for that then you'll be buying trouble if you try to fit a 10 kW one - 'cos the chances of the cabling being suitably rated are minimal.
If you mean can I use ANY MAKE then the answer is probably yes provided it's the same wattage.
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