Your calves being already weaned and eating cake well, I'd just start mixing a rearer cake with their creep and gradually switch them over. The rearer will probably say on the bag/label if there is a minimum age it is suitable for.
Our regular stockblend (16% or 18%) is for cattle 'aged 3 months and older'. I can't get our calves to eat creep, they only want what the mothers are eating
so they all get stockblend right through. But I don't wean at less than 3 months so they only eat a little until then. I start taking mum off at night, and leaving it longer and longer, so they get hungry and eat more cake. Then when they're weaned they know how to eat it and use it to replace the milk they are no longer getting.
(I'm talking my house Jerseys' calves and set-ons; the sucklers get to keep their calves on them until at least 8 months when they're sold, or when winter arrives for those being kept on longer.)