Hi Lucian,
Simplified Answer: Based on that information, I'd give your customers a 24-hour window that could effect their application insights when you plan to make the change.
Although it is seamless it is noted that "Migrating to a workspace-based resource can take up to 24 hours, but the process is usually faster. Rely on accessing data through your Application Insights resource while you wait for the migration process to finish. After it's finished, you'll see new data stored in the Log Analytics workspace tables
".
This process should be seamless as mentioned in the FAQ:
Are there gaps in data collected during migration?
No. We merge data during query time.
Cited from https://learn.microsoft.com/en-us/azure/azure-monitor/app/convert-classic-resource#migration-process
Migration process:
As indicated below, no data is transferred from your classic resource's storage, and this migration process is permanent.
When you migrate to a workspace-based resource, no data is transferred from your classic resource's storage to the new workspace-based storage
. Choosing to migrate changes the location where new data is written to a Log Analytics workspace while preserving access to your classic resource data.
Your classic resource data persists and is subject to the retention settings on your classic Application Insights resource.
All new data ingested post migration is subject to the retention settings of the associated Log Analytics workspace, which also supports different retention settings by data type.
The migration process is permanent and can't be reversed.
After you migrate a resource to workspace-based Application Insights, it will always be a workspace-based resource. After you migrate, you can change the target workspace as often as needed.
Cited from https://learn.microsoft.com/en-us/azure/azure-monitor/app/convert-classic-resource#migration-process
If you require assistance in migration, I'd recommend opening a ticket with Azure Support:
https://azure.microsoft.com/en-ca/support/create-ticket
If this is helpful please accept answer.