geospatial GIS mapping maps

10.4 mxd won’t open in 10.3.1

We recently upgraded to 10.4 and I was trying to share an mxd with a colleague.  They have 10.3.1.  When they try to open it says 10.3.1 can’t open a 10.4 map document.  I haven’t found any way to downgrade.  Does anyone have a workaround?

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