Dave is on the ball, though not all connections are the same they may need some adjustment. Also the board needs to be checked and may require minor upgrade.
Again as Davey rightly said, depends on how old shower is.
There is a difference between a "Power Shower" and a "Pumped Instant Shower" So be specific on the make and model when speaking to a supplier or Plumber.
A power shower only boosts the flow rate where as A pumped instant shower generates its own hot water while also pumping the water.
There are great deals availble from builders merchants on showers at the moment and you will same money by suppling the unit yourself.
There is a firm called shower doctor, they repair and recondition showers too,
you may not need a new shower if its just the heat exchanger, stat or the pump etc, but if its an old shower as in more that 10 years you may be better replacing it.