It will apply to people in the system for ongoing care but not retrospectively.I think if you read the small print it will be for those people that require care after the introduction date ( if it ever gets introduced ) and therefore probably wont apply to people already in the Care Home system. ....that's my guess any way.
At day one everybody should have a local authority care cost which will be update every six months
'Care accounts will be used by local authorities to monitor people’s progress towards the cap. Care account statements will set out both a person’s current rate of progress towards the cap as well as total accrued cost. To ensure people are up to date with their progress towards the cap, care account statements should be sent every 6 months in an electronic format unless an alternative format works better for the individual'
From linked guidance in
https://forum.alzheimers.org.uk/threads/care-homes-costs.150475/post-2079105 .