The tradition of name changing usually is for those who have converted to Sikhi or who have not been raised into Sikhi and did not receive their name according to Maryada. Some examples include Bhai Lehna, Ram Kaur , Madho Das, Nand Lal, and even Bhai Randhir SIngh Ji.
However for some unknown reason this has not always been the case. For example, none of the Panj Pyaarey changed their name nor did Baba Amar Das Ji. Other Gursikhs such as Sujjan Chand , Dayal Das did not change their first name but only changed their last name. I dont know the reason why these Gurmukhs did not change their name despite their hindu background?
If Gursikhs do not consider their name to be legit from a lareedar saroop then those who have married under pad ched saroop would have an invalid marriage, and if they have any children this would be a bajjar kurehit. The panth has not made such claims that ones name or marriage is only legit by lareedar saroop so we shouldnt be so quick to make such claims either.
The simple maryada of selecting name is one first invites sangat . Kirtan is perfromed , ardas is done, Hukumnama is taken, degh and langar is distrubted to attending sangat. THis way baby gets blessed through the blessings of sangat and starts off with a good foot towards Sri Guru Ji. However, I feel like this tradition is being practiced less and less today.