The appearance of legends may have changed when the .mxd file was imported due to improvements in the fitting strategies and changes to legend patch shapes in ArcGIS Pro. Not all legend patch shapes supported in ArcMap are supported in ArcGIS Pro. Unsupported patch shapes have been updated to supported patch shapes.
Before importing the .mxd file to ArcGIS Pro, convert the legend to a graphic in ArcMap. The legend will lose its dynamic properties, but the fitting strategies and patch shapes will be preserved.