Subpanel in the garage is currently fed by 10-2 w/ground romex and is grounded both at the main panel via the ground wire and to the plumbing next to the subpanel (underground iron pipe). I want to relocate the subpanel (still in the garage) and upgrade the feed to 100 amps. I would like to just run a 4-wire feed back to the main panel and bond/ground it there. The original construction was a detached garage but now there is structure connecting the house and garage. Electrically speaking, I don't understand what difference it makes to safety ground attached vs detached. That is, if I consider this to be an attached garage am I missing something since the original design was detached?
There's no longer any difference, aside from the fact that a detached structure needs to be bonded to a separate grounding electrode. In both situations you're now required to run 4 wire cable, and keep the grounded and grounding conductors separate in the second panel.
A detached structure has to be bonded to a grounding electrode, to reduce voltage transients between the structures. This is because the ground "earth" can be at slightly different electrical potentials, even over small distances. Bonding the electrode to the grounding system of the main structure, insures the entire grounding system is at the same potential.