Migrating Users from Internal (AAD) to Guest (AAD)

We're an unusual organisation in that the vast majority of our Dynamics 365 CE Power App users  are outside our organisation.  When the system was implemented, we created AAD accounts in our tenant for these users, and allocated licenses etc.  Not sure why this done (before me!).  We can now provision guest users with the per app licenses and PowerBI licenses that they need, but we have transactions for existing users in Dynamics that we want to preserve, i.e. when we create a guest user for "Person A", we want to ensure that the domain user "Person A"s history is merged (ie, we don't have an audit trail for 2 different user accounts).  So, not an Azure problem, but a Dynamics one.  Perhaps we can somehow update the Dynamics user's SID/object ID from Azure with the "new" one?

Any help or suggestions appreciated.

Steve.