Specify which Migrate entries need to be updated using prepareRow

I have made a Migration that i run with Cron. My highwater field is the date changed, which is present in the XML (source) document. Then I do this in my cron function:

 $result = $migration->prepareUpdate();
 $result = $migration->processImport();

Problem is is this will update ALL nodes from the XML. I would like to only update those that have been changed, according to their modifiedAt date. If I just do:

 $result = $migration->processImport();

Then nothing at all updates. I think processImport() would work if it were just about importing new items, but I also need to update any existing items that have changed. The ones that don’t need updating can be skipped, of course. Is there another method I could call here? Is there a way I need to set up some kind of prepareRow to mark items as needing updating?