Reassign users from old profile to new profile with anonymous Apex

ReassignProfileJob.class

private class ReassignProfileJob {
    
    private String oldProfileName;
    private String newProfileName;
    
    public ReassignProfileJob(String oldProfileName, String newProfileName) {
        this.oldProfileName = oldProfileName;
        this.newProfileName = newProfileName;
    }
    
    public void execute() {
        List<User> affectedUsers = [
            SELECT Id
            FROM User
            WHERE ProfileId = :this.getProfileId(this.oldProfileName)
        ];
        
        Id newProfileId = this.getProfileId(this.newProfileName);
        
        for (User eachUser : affectedUsers) {
            eachUser.ProfileId = newProfileId;
        }
        
        update affectedUsers;
    }
    
    public Id getProfileId(String name) {
        return [
            SELECT Id
            FROM Profile
            WHERE Name = :name
        ].Id;
    }
}

String oldProfileName = 'Old Profile Name';
String newProfileName = 'New Profile Name';

new ReassignProfileJob(oldProfileName, newProfileName).execute();