Flexipay backdates SSP if any changes are made to the SSP calculation. Normally, this happens automatically.
As of version 2.53.0.1 SSP backdating will no longer backdate into a previous year automatically. In the unlikely event that you need to recalculate an old SSP, you can still do it in the SSP diary, but a confirmation will appear if the SSP ends more than 6 months and ends in or before the previous tax year.