That's sound advice, which leads to...
What is more reasonable/practical, is to do NETLIST checking/porting.
That means you can create a NETLIST from (eg) Altium, and import into KiCad, and you can export from KiCad a netlist to verify the design.
These NET files are simple ASCII, and some part-name mapping tables help Lib-Lib. (someone made a great PDF library printing tool for KiCad)
That level may be enough, and I'd certainly suggest you run both CAD tools in parallel anyway, for some months.
Addit: also in the porting/cross-checking basket is this ability in KiCad's GerbView
ie with some caveats, you can load a gerber file, from other tools, into PcbNew - enough to extract XY placement info, and confirm footprint-alignments etc,