What's the difference in MDT to ACM that ACM won't edit an MDT dwg?

When I use AutoCAD Mechanical 2012 to open a dwg created in Mechanical Desktop (idk what version, but I'm thinking 2006 or before...), I get this error:

"You cannot edit Mechanical Desktop drawings in AutoCAD Mechanical"

"All AutoCAD Mechanical commands, including save are now Disabled"

I am not new to AutoCAD, but am new to the Mechanical series. I've read threads dating back to 2006 that say 'Mechanical Desktop 3D drawings cannot be edited in AutoCAD Mechanical. This is by Design and has always been like this in the past too.', but I want to know WHY and how can we use the old files?? It seems like a waste if we can't use the library it took years to build...

The most pressing issue is that we cannot 'snap' to anything in the dwgs. We need to use these previous dwgs to get dimensions and other info for new projects. Can we convert or upgrade the old dwgs in some way to be able to use them in AutoCAD Mechanical 2012.

Thank you for any insight into this issue.