Retain Brendon McCullum as an opener. That is the only conclusion that can be drawn even after the diminutive dasher copped a lot of the flak for the Black Caps' weak showing against a vulnerable Australia in the first test.
First reason: There isn't anyone else. Second reason: In his 12 test digs as an opener since November 2010, he is averaging 48.90 against major test nations. That includes that glorious 225 against India in Hyderabad last year and, if you except that, his average is 31.3. Only ... who on Earth is going to exempt a double test ton against India?
In his 12 test innings at the top of the order, he has scored three 50s or better and that double century in 11 knocks. That is his record against major test-playing nations like Australia and India with minnows like Bangladesh weeded out.
If you take the same measure through his career as a bat down the order, his previous 12 test innings (against India, Australia, Pakistan and Sri Lanka; with minnows excepted) shows him hitting 462 runs at an average of 38.5 (one century, three 50s or better).
Extrapolate that through the last three years and, against major test opposition, McCullum scored 1275 runs (two centuries and eight half centuries), at an average of 33.55 across 38 test innings as a lower order batsman. So, with not much more than a quarter of the innings as an opener as he played down the order in the last three years, McCullum is ahead of the curve.