When you first start diaper training your girlfriend or wife, letting them wear their old underwear over their new underwear is acceptable, but only during the first stage. Wearing panties over their pull-ups makes the pull-up feel more normal, and after a few weeks of this, they’ll come to a point where they’ll have no option but to use their pull-up. She’ll soon discover that wearing protection all the time isn’t so bad, so she’ll choose to keep wearing them for those ‘just in case’ moments.
After that, you can suggest that she should throw away her old panties, and you must let her make that decision herself. If you throw away her panties, there’s a high chance that she’ll reject her diaper training, so it is imperative that she must choose to remain in diapers, and when she inevitably does chose to wear protection from now on, you must shower her in love and treat her like the padded princess she is.