Every retention tag has a tag name, retention age and retention action ,each tag also has a unique GUIDs.
[PS] C:\>Get-RetentionPolicyTag "Inbox-365days-Delete"|FT GUID,RetentionID
When MFA processes emails and tags emails with tag and expiration information, it also populates the RetentionId of the tag in the PR_POLICY_TAG or PR_ARCHIVE_TAG property. When Outlook in cache mode sync the tagged email, the tag information is also synced in the OST.
For some reason if you change the policy on the mailbox which has new set of tags, MFA is going to re-evaluates all the emails in the mailbox again ,replace the RetentionId in PR_POLICY_TAG or PR_ARCHIVE_TAG property and recalculate the expiration information. Because of this change on the emails properties, Outlook is going to resync these emails in the OST again.
Outlook performance might degrade until the complete OST sync is complete. User connecting remotely might get affected, due to OST sync and if the policy was changed for large number of users, we might see network bandwith getting consumed when large number of users are syncing their OST.
This is noticable if policy is changed on bulk users, Outlook users are accessing their mailbox remotely in cache,mailbox size is large and remote connectivity is poor.
What happens, when moving mailboxes to another forest or Office 365.?
Technet has published a script which exports retention tags from source Exchange organization to xml file and imports tags from the xml file to target Exchange organization.This process is pretty simple.
When the script creates tags in target organization, these tags will have the same RetentionID as the source. Thus OST syncing is avoided.
In on-premise the guid in GUID and RetentionID are the same, whereas they are different for tags created in Office 365.
Therefore when the script is used to import tags in the Office 365 ,the imported tag will have a unique GUID and the same RetentionID as on-premise.
You cannot change the GUID to match the RetentionId. It is not required, RetentionId is one which needs to same as on-premise.
On-premise tag information.
After importing the tags using the script.
Above we see that the RetentionId remains the same for imported tag. Having the same RetentionId is the key to prevent OST from resyncing.
Hope this help!