Issue changing retention period in Exchange 2010

We are trying to change the default retention period that is currently applied to all email in our Exchange 2010 server.   It was originally set for 10 years, 3650 days, and we are changing that to 2 years, 730 days.  We have made the change, however;  all old items and even new items are still marked with the 10 year retention time.  How do we get the messages to take the new 2 year retention period; even those messages previously marked for the longer retention time?  Also, why aren't the new messages being marked with the new 2 year retention period?  Any help is appreciated.
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TPBPITAuthor Commented:
I figured out the issue.  The amount of items that had to be dealt with was so great, that it took several round of it running to clean it up.  Thanks for the posts.
timgreen7077Exchange EngineerCommented:
There is a bit more to know about retention policies, and there can be a few different reason why the policy has been taken affect or been applies because it can depend on the type of policy that is currently set or being applied, or did the Managed Folder assist run yet, and Exchange throttles retention actions so that it doesn't affect Exchange performance too much so not all policies will apply at the same time unless you force it to. So look at the reference links and reach out if you still have questions:
Prashant ShrivastavaConsultantCommented:
The "clock" starts when a tag is first applied to an item. So when you first apply a retention policy to a mailbox/folder, items that have never been tagged before will have their clock start then. At least I know this to be the case with the Deleted Items folder. It may actually work differently in other folders. I haven't gone back to test further with other folders.

I've got a post on this behavior here:
TPBPITAuthor Commented:
Because we solved our own problem.
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