would I be able to put new rear wheel with 12mm x 142 axle on my 2007 RM Element 70 frame?
The Element 70 has a 10mm x 135mm QR rear hub so the rear dropout will not be wide enough to fit a 12mm x 142mm wheel. The 12mm is the axle diameter, the 142mm the dropout width. New rear hub standards are not only larger diameter axles but wider hubs, front hubs are different in that you can use a thru axle with a shim to fit a QR dropout as the width did not change.