Well, the priesthood was not based on, established by, the Levitical law (7:11),
it was established by ordination of Moses.
There is no Levitical law establishing the priesthood.
Rather, the law was based on the priesthood (7:11).
The "change of the law" being spoken of is the change of the Mosaic law (7:18-19)
which was based on the Levitical priesthood (7:11),
which priesthood was removed, necessitating a change of the Mosaic law on which it was based.