jvn, I am pasting the reply I got from immigration to this same query:
If both you and your spouse are not HK permanent residents, the form ID235B was issued with your child's HK birth certificate by the Birth Registry to indicate his stay in HK.
If your child has a separate travel document now, you may consider transferring the visa label from his/her ID235B to his/her new passport.
Transfer of endorsement:-
Application shall be submitted to the Extension Section, 5/F Immigration Tower, 7 Gloucester Road, Wan Chai, Hong Kong with the following documents:
(i) applicant's birth certificate (original and copy);
(ii) applicant's travel document (original and copy);
(iii) copy of sponsor's (father/mother) travel document;
(iv) copy of parents' marriage certificate;
(v) form ID235B
The fee for application for transfer of an endorsement to a travel document is HK$165.
In general, the baby no need to attend in person and the transfer of endorsement can be completed within the same day. However, it requires the baby to be in HK when the application is submitted and issue of the endorsement.
So basically, once you have your passport, all you need is one day to transfer this sticker. I'd encourage you to email Immigration directly. They reply within TWO HOURS and these are proper replies not the automated ones. I am a huge fan of HK Immigration!