Reversing and resyncing a SnapMirror relationship takes a number of steps...roughly, those are:
Check for common snapshots between source and destination.
Remove the existing SnapMirror relationship (use the Remove-NcSnapmirror cmdlet)
Remove snapshots from the original source volume (on CL1 in your example) which are newer than snapshots on the original destination (CL2).
Recreate the SnapMirror relationship (use the New-NcSnapmirror cmdlet) in reverse, where CL2 is the source and CL1 is the destination. It should automatically use the newest common snapshot as the baseline.
Resync the the SnapMirror (use the Invoke-NcSnapmirrorResync cmdlet).
Hope that helps.
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