I found this and it solved the problem: Once Powershell is open and connected to the Office 365 service, run the following line:
Set-RetentionPolicyTag "Deleted Items" -AgeLimitForRetention 730
That would extend the period that items are kept in the Deleted Items folder from the default 30 day period to 2years (730 days).
Seems to work, and since this amends the default policy tag, it applies to all mailboxes without having to apply it individually.