change of boiler

Based on the limitations of your question............. A competent gas engineer should carry out a gas pipe sizing exercise first. This involves determining how many gas appliances you have the kilowatt rating of each appliance and the size of the existing gas carcase (where reasonably practical) Once all this information has been processed the engineer will determine the size of gas pipe required to serve your new boiler which MAY require up sizing from the meter. If a boiler (or any other gas appliance) is starved of gas because the pipe is under sized resulting in low pressure this can create big problems. Recommend you ask another Gas Safe engineer to attend who knows what actually is required as described above!

You haven't entered enough information about your new boiler to be able to answer the question. If its a Worcester' 12kw heat only it may be adequate, your installer will fit an adequate gas supply as part of the installation.