When I got my girls, I wanted to switch them to harringtons. I did do it slowly but they refused to eat the harringtons. I did stop and start again, second time round being more persistent. I just kept decreasing the amount gradually as advised then stopped giving them the burgess when it ran out. They did eventually start eating the harringtons.
I would just persist and keep offering the new ones as you gradually decrease the amount of P@H pellets. They’re small part of their diet (5%) so it wouldn’t worry me too much - as long as they are getting plenty of hay and a varied diet with vitamin c veg - coriander and bell pepper.