Poole hospital will lose its emergency department in July/ August 2026, Mr Donald said.
He added that it will it instead becomes the “largest planned care hospital site in the country”.
It will mean people taken by ambulance for emergency treatment will go to the hospital in Bournemouth, while patients who have planned procedures, walk-ins and non-emergency healthcare services will go to the hospital in Poole.
The two hospitals are 8.4 miles (13.5 km) apart.
Mr Donald said it is hoped that the changes will help to reduce waiting times for patients.
“If we’ve got a centre dedicated to planned care then the point of this is things won’t be bumped by emergency pressures,” he said.
He acknowledged that it is a very large change but said they are already “seeing better results” in services like Poole’s maternity unit, which moved to Bournemouth earlier this year.