複数の version がインストールされている Formula を切り替える ref: http://qiita.com/iorionda/items/14d4ff8df7ba2c4bf178

file0.txt

% brew switch <formula> <version>

file1.txt

% brew help
Example usage:
  brew [info | home | options ] [FORMULA...]
  brew install FORMULA...
  brew uninstall FORMULA...
  brew search [foo]
  brew list [FORMULA...]
  brew update
  brew upgrade [FORMULA...]
  brew pin/unpin [FORMULA...]

Troubleshooting:
  brew doctor
  brew install -vd FORMULA
  brew [--env | config]

Brewing:
  brew create [URL [--no-fetch]]
  brew edit [FORMULA...]
  open https://github.com/Homebrew/homebrew/blob/master/share/doc/homebrew/Formula-Cookbook.md

Further help:
  man brew
  brew home

file2.txt

% brew switch <formula> <version>

file3.txt

% convert --version
Version: ImageMagick 6.8.9-8 Q16 x86_64 2014-11-11 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2014 ImageMagick Studio LLC
Features: DPC Modules
Delegates: bzlib freetype jng jpeg lcms ltdl lzma png tiff zlib

file4.txt

% brew info imagemagick
imagemagick: stable 6.8.9-8 (bottled), HEAD
http://www.imagemagick.org
/usr/local/Cellar/imagemagick/6.8.8-9 (1433 files, 22M) *
  Built from source with: --with-libtiff
/usr/local/Cellar/imagemagick/6.8.9-1 (1436 files, 22M)
  Built from source with: --with-libtiff
/usr/local/Cellar/imagemagick/6.8.9-8 (1437 files, 22M)
  Built from source with: --with-libtiff
From: https://github.com/Homebrew/homebrew/blob/master/Library/Formula/imagemagick.rb

...

file5.txt

% brew switch imagemagick 6.8.8-9
Cleaning /usr/local/Cellar/imagemagick/6.8.8-9
Cleaning /usr/local/Cellar/imagemagick/6.8.9-1
Cleaning /usr/local/Cellar/imagemagick/6.8.9-8
71 links created for /usr/local/Cellar/imagemagick/6.8.8-9

file6.txt

% convert --version
Version: ImageMagick 6.8.8-9 Q16 x86_64 2014-04-16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2014 ImageMagick Studio LLC
Features: DPC Modules
Delegates: bzlib freetype jng jpeg lcms ltdl png tiff xml zlib