change GET query string

I'm not sure if this is what you want, but you can use codeigniter ability to parse uri
uri_to_assoc();
http://whatever.com/brand/foo/model/bar
(
        'brand' => 'foo',
        'model' => 'bar',
)

assoc_to_uri()
(
        'brand' => 'foo',
        'model' => 'bar',
)
http://whatever.com/brand/foo/model/bar
assoc_to_uri()
(
        'model' => 'bar',
)
http://whatever.com/model/bar

But if you really need exactly the query string, there could be 'http_build_query()' usefull for you.
When you build response, just create array of parameters and uset all 'empty' keys. Then 'http_build_query()' should create query without 'brand' if it is empty.