No, it is never too late, retrofitting it just requires some lead-time and such.

Bitcoin retrofitted primary-chain-only merged mining support long ago, other chains retrofitted full merged mining support. It can be done. Some people fear it can devalue a coin but the energy savings should be more worth it for a coin that purports to actually be interested in energy savings than it was/is for any other coin that "bit the bullet" and took that risk - or that actual hit to value even - for the good of the planet and the environment and all that good stuff.

-MarkM-