Hello Hans,
The only option I know of is to adjust the currently used retention policy. I do not know if this is a retention you apply to all users.
Example.
Retention 1 = indefinite (all users)
Retention 2 = 10 years (specific group of users)
Change retention 2 to be 5 years. This will also apply to older items. (because the retentioncategory is stored, not the actual value)
Not sure if this answers your question, and as alwyas, test!
Regards. Gertjan