Tested on Version 12.0.2503.10378
This is the example given in the help documentation:

LFForm.changeFieldOptions( { fieldId: 10 }, [{label: "Choice 4", value: "Choice_4", selected: true}, {label: "Choice 5"}], "add" );

I've been playing around with this, and it doesn't look like including the "selected: true" element works with drop-down fields.  It adds the value(s) to the drop-down, but does not select the value no matter what I do with my testing.

Is this a bug?