This will be an amendment to the Dangerous Dogs Act 1991
As for what happens to existing dogs - from Wikipedia -
The process for getting a Section 1 dog exempted includes proving to the court that the dog is not a danger to public safety, that it is owned by a 'fit and proper' person to be in charge of a dog, that the dog is already neutered and microchipped,[Note 1] and that the owner has obtained third-party insurance that would cover an incident of bodily injury or death of a person caused by the dog. Ongoing conditions include keeping the dog at the address listed, notifying of address changes, notifying of the death or export of the dog, keeping the dog muzzled and on a lead in public places, keeping the dog securely to prevent escape, and maintaining all previous conditions for the life of the dog.[18]
The Act established the Index of Exempted Dogs and the Animal Welfare section of the Department for Environment, Food and Rural Affairs (Defra) oversees the administration of the Act and the Index.[16]
Initially, dogs born before 30 November 1991 were eligible to be put on the Index of Exempted Dogs (a grandfather clause). Applications were received for over 8,000 dogs; 5,223 dogs received their Certificate of Exemption.[19] Dogs born after 30 November 1991 were not eligible to be on the Index, and it was expected the Index would cease after the death of the last of the original 5,223 dogs. However, the 1997 amendments expanded eligibility, effectively continuing the Index. As of 2015, there were 3,001 Pit bull terriers on the Index, 6 Dogo Argentinos, 0 Fila Brasilieros, and 3 Japanese Tosas.[20] As of 2018, there were 3,514 Pit bull terriers, 3 Japanese Tosas, 13 Dogo Argentinos, and 0 Fila Brazilieros.[21]
It will be interesting to see how many pass the 'fit and proper person' test and how many pass the temperament test.