The answer to your question has nothing to do with your new boiler being too powerful, but depends on whether your microbore system provided adequate heating in the rooms previously. If the microbore system was adequate before, it will be adeqate still when connected to the new boiler assuming the original boiler had sufficient heating output to serve the radiators. A good heating engineer should be able asses this correctly rather than the rather stupid recommendation you have been given. If you are extending the heating system to the loft rooms, It will be an easy matter to run an additional circuit to this area direct from the boiler to avoid overloading existing pipework.
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It is also an old plumbers tale that microbore system silt up. This can only happen if internal corrosion is occurring or fresh water carrying calcium and other deposits is entering the system to replenish leaks and other system faults. If the system was properly installed and has been well maintained the system water will be reasonably clean if not rather black in colour. Check the requirements of the new boiler manufacturer's. They may require a power flush or just specify a good flush with a special central heating cleaner.
My first impressions of your proposed installer are not good and recommend you get a second qualified opinion.