Retrofit to existing or complete replacement
Solar geysers for which you pay a premium are required to have a better standing loss performance than a standard geyser.
The SABS spec states that a 200L Solar geyser must achieve 2.26 kWh or better. That’s only 0.04kWh better than a popular brand of standard geyser. A good argument in itself to uses a system that will fit to your existing geyser rather than buy a new one.
In terms of losses there is often no significant difference between a solar geyser and a good standard one with adequate insulation. Greater losses come from un-insulated pipe work.
Almost invariably it’s the suppliers who market integrated systems (though with built in tanks ) that decry the use of existing geysers.