If your considering changing your fireplace that is connected to a back boiler system then you should consider two main strategies:
Keep the back boiler and change fireplace:
The first obvious option is to simply change the fireplace (decorative part) and retain the working back boiler. The main thing you must ensure is that the new fire is compatible with the back boiler as some boilers have been specifically designed to work with a limited range of inset Fires. Back boilers are almost obsolete now and there is a limited range of manufacturers making replacement Fires for them as a result. If you are retaining the back boiler system, then you should also ensure that the overall height of the new fire matches the height of the back boiler system.
Replace fireplace with obsolete removed back boiler:
If your back boiler needs replacement then it is very easy to remove the current boiler and replace the fireplace to something that is more aesthetically pleasing and complimentary to the room. Typically the back boiler will have ample depth and hence you can install virtually any fireplace you like in the required area (as there is enough recess depth). The fireplace will also have a gas connection strategically located (previously used for the boiler) and effectively you can install a full depth convector gas fire.