Post any defects you find in the released or beta versions of the ImageMagick software here. Include the ImageMagick version, OS, and any command-line required to reproduce the problem. Got a patch for a bug? Post it here.
-alpha activate/deactivate
enables and disables the alpha channel, respectively, with persistence. This is like on/off in Imagemagick 6. In Imagemagick 7, -alpha off will remove the alpha channel permanently such that -alpha on will not re-enable it.
Understood about the colorspace. IM 6 uses Rec601luma and IM 7 use Rec709luma. So you meant to say use -intensity Rec601luma to make IM 7 reproduce IM 6 results.