Selgas
Firstly lets go from the end of your question to the front cost to change over if it is an external model would be in the vacinity of $1500 or so as it is a simple task. If however, your unit is an internal one then depending on the flue presently fitted and a couple of other issues this could end up costing upwards of $2500 – depends on too many variables that we have not been able to view.
If your water heater is a thermostatic model then I would suggest that the thermostat file be checked firstly and if it has failed a replacement would overcome your current problem, if it tests to be sound then a full service of both the gas and water valve assemblies would have the same result.
