Some more test results. Bottomline: `brew cask` does try to clean up the legacy `phinze/cask` tap, but it seems to do it poorly and leaves the `caskroom/cask` tap in a dirty state where the `brew-cask` package is not longer found.

I'd recommend taking my original change where it checks for the presence of the old `phinze/cask` tap and removes it before installing the new `caskroom/cask` tap.

Steps to reproduce:

Let's say I already have the old cask installed:
```
➜  ~  brew tap
phinze/cask
```

Now we install `caskroom/cask` over it without first removing `phinze/cask` (essentially option 2 above):

```
➜  ~  brew tap caskroom/cask
Cloning into '/usr/local/Library/Taps/caskroom/homebrew-cask'...
remote: Reusing existing pack: 31334, done.
remote: Counting objects: 12, done.
remote: Compressing objects: 100% (12/12), done.
remote: Total 31346 (delta 4), reused 1 (delta 0)
Receiving objects: 100% (31346/31346), 11.45 MiB | 2.41 MiB/s, done.
Resolving deltas: 100% (18331/18331), done.
Checking connectivity... done.
Warning: Could not tap caskroom/homebrew-cask/brew-cask over phinze/homebrew-cask/brew-cask
Tapped 1 formula
```

We then run `brew cask search` to let cask clean up the legacy tap:

```
➜  ~  brew cask search xxxxx
==> Removing legacy Tap
No cask found for "xxxxx".
```

It does appear to have cleaned it up:

```
➜  ~  brew tap
caskroom/cask
```

However, it leaves the tap in a dirty state. One symptom - `brew-cask` is not longer found:

```
➜  ~  brew info brew-cask
Error: No available formula for brew-cask
```

I have to re-tap `caskroom-cask` to get it back to a clean state:

```
➜  ~  brew untap caskroom/cask
Untapped 1 formula
➜  ~  brew tap caskroom/cask
Cloning into '/usr/local/Library/Taps/caskroom/homebrew-cask'...
remote: Reusing existing pack: 31334, done.
remote: Counting objects: 12, done.
remote: Compressing objects: 100% (12/12), done.
remote: Total 31346 (delta 4), reused 1 (delta 0)
Receiving objects: 100% (31346/31346), 11.45 MiB | 1.46 MiB/s, done.
Resolving deltas: 100% (18331/18331), done.
Checking connectivity... done.
Tapped 1 formula
➜  ~  brew info brew-cask
brew-cask: stable 0.35.0, HEAD
https://github.com/caskroom/homebrew-cask/
/usr/local/Cellar/brew-cask/0.35.0 (1565 files, 6.2M) *
  Built from source
From: https://github.com/caskroom/homebrew-cask/commits/master/brew-cask.rb
```