Complex sentence: The original local name would have been "Dorn-gweir" giving the Old English "Dornwary", and the name changed over time to Dorncester/Dornceaster and Dorchester.
Split into simpler sentences: The original local name would have been "Dorn-gweir" giving the Old English "Dornwary". This name evolved over time to Dorncester/Dornceaster and Dorchester.