Give or take loss in fittings, the 'pressure drop' in the old pipe will be so many millibars per metre of length. So, if you only have a short distance of the smaller pipe left, before you join onto your new larger diameter pipe (which loses less pressure per metre), that will still help.
When we did similar, we used a plumber who was *registered with* Scottish Water. He checked over the pipe we had laid, in the trench we had dug, gave it the OK and then made the connection to the mains. This was substantially cheaper than having it done by Scottish Water themselves.