Yes, the same lens is used to read both the DVD and BR-D, however, the bandwidth of the laser beam is changed on a media basis.  When you put a disc in, it searches, in a certain order like; CD, DVD, BR-D; as the laser and lens adjust accordingly.   When it is able to detect the appropriate media, it locks in that bandwidth for the media being used.

Originally, a CD drive laser was RED, later they went blue with the same width as a red laser.   DVD lasers are also BLUE.  The wavelength (beam width) is what allows the unit to read more densely packed lands and pits...  What is changed is how the stepper motor moves the lens assembly.  The pulse sent to the stepper motor is based on the media, a LARGER pulse to move the head out for CDs, and the pulse narrows with more densely packed media...

A BR-D drive can read HD-DVD discs if the firmware allowed it...  All multi read units use the same hardware, different focusing and stepper pulses...  Same principal as the floppy drive, but with an optical head -vs- magnetic head.  HOWEVER, you can use an optical head to READ a floppy, just not write one...   It's all quite fascinating if you take the hour or two to read up on how it all works.

SCRATCHES cause the lens to be moved up and down constantly to try and focus on the data.  Radial scratches are less harmful than circular ones, because the laser may (and they do)  track the scratch  if it follows the contour of the disc.  That is why, when you clean a disc with a cloth, you wipe from the hub, directly out to the edge.  Never clean a disc in a circle... 

Anyhow, no, scratches do not prematurely burn out a laser diode....