The Post Office said the changes would help to keep branches where customers wanted and needed them to be.
But the Communication Workers' Union (CWU) said the move was "evidence that the Post Office is in crisis".
The union added that the number of post office jobs lost since the beginning of last year was 2,000.
The CWU held a five-day strike before Christmas at Crown post offices in a dispute partly about branch closures. The union is also concerned about job losses and pension provision.
Crown post offices are the larger branches usually found on High Streets.
In a statement, the Post Office said: "With consumer habits changing, and the high cost of maintaining premises in prime high street locations, franchising helps us to keep services where our customers want and need them."
It said it would take time to identify the right partners over the coming months and all proposals would be subject to local consultation. It promised to keep staff "fully informed".
CWU general secretary Dave Ward said: "The latest round of closures is further evidence that the Post Office is in crisis and that the board of the company, backed by the government, is simply pursuing a strategy of slash and burn.
"Today's announcement comes less than three weeks after the closure of a major government consultation on the future of the Post Office and sticks two fingers up to everyone who took part in this."
The union said that 75,000 postcards had been sent by members of the public to the government calling for an end to the closure and franchise programme of the Post Office.
It added that the Post Office network has been reduced by more than 50% over the past 30 years.
In April 2016, the Post Office announced plans to transfer up to 61 branches into WH Smith stores over the following year.
It said the move was part of a 10-year plan to cut costs and save cash, and would act as a way of "safeguarding the future of the network".

Mr Maskey took 70.6% of the vote in Thursday's by-election with 16,211 votes. The turnout of 37.53% was one of the lowest for Northern Ireland.
In second place was the SDLP's Alex Attwood on 3,088, with Gerry Carroll of People Before Profit third on 1,751.
The by-election was held after Sinn Fein president Gerry Adams resigned earlier this year to become a member of the Republic of Ireland's parliament.
Brian Kingston of the DUP took 1,393 votes, Bill Manwaring of the UUP 386 and Aaron McIntyre of the Alliance Party 122.
The result was seen by many as a foregone conclusion as it is the safest seat in Northern Ireland, and the fourth safest in the UK.
The full turnout of 37.53% drops to 37.35% when spoiled votes are excluded. The previous record low was 38.75% at North Down in 1995.
Mr Maskey, who is a sitting assembly member and a long-time Belfast city councillor, was also celebrating his 44th birthday.
He said he was not concerned by the turnout, adding that people had been asked to cast three separate votes just five weeks ago for the Northern Ireland Assembly, local government and AV referendum.
He said: "I am delighted to be elected by the people, they have endorsed Sinn Fein yet again for their Westminster seat."
'Strong legacy'
Accompanied by wife Patricia and his brother, fellow Sinn Fein MLA Alex Maskey, he paid tribute to his predecessor as he vowed to improve the lives of people in a constituency that has some of the most deprived neighbourhoods in Northern Ireland.
"This part of the city is moving forward and I will endeavour to be part of that and work tirelessly to do what I can for the people of west Belfast," he said.
"I take on the proud and strong legacy of Sinn Fein party president Gerry Adams and look forward now to the challenges in front of me."
Mr Adams presented his successor with a special hurley to mark the result as other supporters urged him to blow out the candles on a birthday cake.
"It's Paul Maskey's night," said the Sinn Fein president.
"Once again Sinn Fein in west Belfast has delivered and we delivered big time."
Sinn Fein's MPs have not take their seats at Westminster because they refuse to swear an oath of allegiance to the Queen.
The Post Office is planning to close and franchise 37 Crown offices, with, the union says, the loss of 300 jobs and 127 specialist roles.
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Sinn Fein's Paul Maskey has been elected as the new MP for West Belfast.