Charles Ellicott Commentary Leviticus 27:33

"He shall not search whether it be good or bad, neither shall he change it: and if he change it at all, then both it and that for which it is changed shall be holy; it shall not be redeemed." — Leviticus 27:33 (ASV)

He shall not search whether it be good or bad. —That is, the owner is not to pick out the good ones from the bad, but, as described above, is to mark every tenth one as it comes out of the fold as belonging to the Lord.

And if he change it at all. —See Leviticus 27:10.