The mousemap entry that enables command+click is this one:
{
"button": "button1", "count": 1, "modifiers": ["super"],
"press_command": "drag_select",
"press_args": {"additive": true}
}
You’d only need to create your User/Default (OSX).sublime-mousemap file and change "super" to "alt" to change it.
I would advise to include alt versions of all three of the first sections (Basic drag select, Drag select by words, Drag select by lines). Keep in mind this will disable the column-select behavior of the Alt + Mouse 1 Column select section unless you modified those as well.