The ID Service Grace Period
After you deploy the Experience Cloud ID service, new visitors no longer receive an Analytics visitor ID from your data collection server. If sections of your site have not yet implemented the Experience Cloud ID service, when visitors browse to these sections, the Experience Cloud ID is not recognized and visitors are assigned a legacy Analytics visitor ID. This can create duplicated visit counts and incorrect attribution.
Grace period support requires version 1.3 or later of the Experience Cloud ID service.
Do I need a grace period?
same report suite, or if you are using other technologies on your site such as Flash video measurement, we recommend configuring a grace period.