Guidance published today reveals that the 15-month transition period began last Friday (29 June).
Councils and their safeguarding partners will have up to 12 months to agree the new arrangements, and three months to implement the changes. The final deadline date is 29 September 2019.
Once the arrangements have been published and implemented, the LSCB for the local area will be given a "grace" period of up to 12 months to complete and publish outstanding SCRs and of up to four months to complete outstanding child death reviews.
The guidance states that LSCBs must complete all child death reviews by 29 January 2020 and all SCRs by 29 September 2020 at the latest. Once outstanding SCRs and child death reviews have been completed, the LSCB will cease to exist.
The government has also announced that 17 areas of the country, covering 39 local authorities, will act as "early adopters", working with the National Children's Bureau to implement the new arrangements before they are established across the rest of the country.
The early adopters will be tasked with "developing new and innovative approaches to set up multi-agency safeguarding processes and produce clear learning which can be shared across other areas".
Local authorities have been required to have an LSCB since April 2006, under provisions of the Children Act 2004. Plans to replace the current system of LSCBs and SCRs with new arrangements were drafted on the back of a review conducted by Sir Alan Wood, which was published in May 2016.
Instead of LSCBs, there will be a requirement on three partners - local authorities, the police and the health service - to make arrangements for working together on child protection in a local area.
Concerns have previously been raised that the changes could risk "confusing and distracting" the sector. But the DfE said the response to the draft guidance around the changes was "generally positive".
The transitional guidance will sit alongside statutory Working Together to Safeguard Children guidance, which has also been updated to reflect the changes.
Children's minister Nadhim Zahawi said: "We all have a responsibility to promote the welfare of children and protect those at risk of harm. It is important that young people can grow up in an environment that is as safe and stable as we would want for our own children.
"That's why we have changed the law to create a stronger safeguarding system, placing greater accountability on the key professionals involved so vulnerable children can get the support and protection they deserve.
"This guidance will bring health agencies, police forces and councils together to work more collaboratively, making effective decisions that put the needs of local families at the heart of their work."
Carolyne Willow, director of children's rights charity Article 39, said she was disappointed that the government has not taken the opportunity to strengthen children's rights in the new guidance, something it had said it would consider when it published its response to the proposed Working Together changes in February.
"Back in February the government said it would be considering the emphasis given to the child's voice and children's rights during the revision process," Willow said.
"If such changes have been made, they are imperceptible.
"Crucial content around keeping the child in focus, children's rights and advocacy services was culled from the guidance in 2010. We had high hopes that some of this would reappear, especially because this is the first revision since the UK's biggest public inquiry into child sexual abuse started its work.
"The guidance remains weak when it comes to investigating institutional abuse allegations, and supporting children in those settings who have been mistreated. We will continue to lobby for distinct statutory guidance for this particularly vulnerable group of children."