What does SW do to older version files when opening in newer version?

OK, we've finally made the jump from SW2010 up to SW2015 and I'm wanting to know what SW does to the old version files (that are in my PDM vault)?  The majority of times my users will not be checking out many of the files in an Assembly so I don't think those files get converted and placed back into the vault.  Is there a way, besides Task Scheduler, to update the files in the vault?  BTW, we are using Smarteam

TIA,

Steve

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