Keep back boiler fire and fit a combi boiler

This is possible as long as you understand that the existing back boiler will be de-commissioned so that it can no longer function - also it requires to be made safe which will be part of the de-commissioning process. Modifications to the heating system and pipework will be required to facilitate a pressurised heating system. Thanks for the update. When you say 'Plumber' I hope you mean Gas Safe engineer? I cannot comment blind on your gas fire, but what has been proposed as long as the flue way integrity of the fire has not been compromised and all the products of combustion can be safely discharged to atmosphere and no additional ventilation is required to serve your fire, then in principal it can be achieved, but if in doubt ask another Gas Safe engineer to provide a second opinion.