It should be merely a "rebuild", but all source packages would need to be modified: Since the ABI of the libs is also (as is the case for this bug) dependant on the haskell compiler used, all (library) packages have quite strict dependencies on the particular ghc6 version. Furthermore, these "rebuilds" will need to be done in order, so libraries build-depending on other libraries need to get updated to reflect the new version of the build-dependency.

I don't imagine there should be any problems due to this change in ghc6 so that packages won't build again if they previously did. However haskell allows to mix native calls with haskell code, so it's in theory possible.

Binary packages, that would be affected:
$ apt-cache rdepends ghc6
  libghc6-xmonad-dev      
  libghc6-xmonad-dev      
  libghc6-xmonad-contrib-dev
  libghc6-xmonad-contrib-dev
  libghc6-xhtml-dev         
  libghc6-xhtml-dev         
  libghc6-x11-dev           
  libghc6-x11-dev           
  libghc6-wash-dev          
  libghc6-wash-dev          
  libghc6-vty-dev           
  libghc6-vty-dev           
  libghc6-uulib-dev         
  libghc6-uulib-dev         
  libghc6-utf8-string-dev   
  libghc6-utf8-string-dev   
  libghc6-time-dev          
  libghc6-time-dev          
  libghc6-tagsoup-dev       
  libghc6-tagsoup-dev       
  libghc6-stream-dev        
  libghc6-stream-dev        
  libghc6-stm-dev           
  libghc6-stm-dev           
  libghc6-src-exts-dev      
  libghc6-src-exts-dev      
  libghc6-sourceview-dev    
  libghc6-sourceview-dev    
  libghc6-soegtk-dev        
  libghc6-soegtk-dev        
  libghc6-regex-posix-dev   
  libghc6-regex-posix-dev   
  libghc6-regex-compat-dev  
  libghc6-regex-compat-dev  
  libghc6-regex-base-dev    
  libghc6-regex-base-dev    
  libghc6-quickcheck-dev    
  libghc6-quickcheck-dev    
  libghc6-plugins-dev       
  libghc6-plugins-dev       
  libghc6-pcre-light-dev    
  libghc6-pcre-light-dev    
  libghc6-parsec-dev        
  libghc6-parsec-dev        
  libghc6-parallel-dev      
  libghc6-parallel-dev      
  libghc6-pandoc-dev        
  libghc6-pandoc-dev        
  libghc6-opengl-dev        
  libghc6-opengl-dev        
  libghc6-openal-dev        
  libghc6-openal-dev        
  libghc6-network-dev       
  libghc6-network-dev       
  libghc6-mtl-dev           
  libghc6-mtl-dev           
  libghc6-missingpy-dev     
  libghc6-missingpy-dev     
  libghc6-missingh-dev      
  libghc6-missingh-dev      
  libghc6-magic-dev         
  libghc6-magic-dev         
  libghc6-listlike-dev      
  libghc6-listlike-dev      
  libghc6-ldap-dev          
  libghc6-ldap-dev          
  libghc6-irc-dev           
  libghc6-irc-dev           
  libghc6-hunit-dev         
  libghc6-hunit-dev         
  libghc6-http-dev          
  libghc6-http-dev          
  libghc6-html-dev          
  libghc6-html-dev          
  libghc6-hsql-sqlite3-dev  
  libghc6-hsql-sqlite3-dev  
  libghc6-hsql-postgresql-dev
  libghc6-hsql-postgresql-dev
  libghc6-hsql-odbc-dev      
  libghc6-hsql-odbc-dev      
  libghc6-hsql-mysql-dev     
  libghc6-hsql-mysql-dev     
  libghc6-hsql-dev           
  libghc6-hsql-dev           
  libghc6-hspread-dev        
  libghc6-hspread-dev        
  libghc6-hslogger-dev       
  libghc6-hslogger-dev       
  libghc6-hsh-dev            
  libghc6-hsh-dev            
  libghc6-hlist-dev          
  libghc6-hlist-dev          
  libghc6-highlighting-kate-dev
  libghc6-highlighting-kate-dev
  libghc6-hgl-dev              
  libghc6-hgl-dev              
  libghc6-hdbc-sqlite3-dev     
  libghc6-hdbc-sqlite3-dev     
  libghc6-hdbc-postgresql-dev  
  libghc6-hdbc-postgresql-dev  
  libghc6-hdbc-odbc-dev        
  libghc6-hdbc-odbc-dev        
  libghc6-hdbc-missingh-dev    
  libghc6-hdbc-missingh-dev    
  libghc6-hdbc-dev             
  libghc6-hdbc-dev             
  libghc6-haxml-dev            
  libghc6-haxml-dev            
  libghc6-hat-dev              
  libghc6-hat-dev              
  libghc6-haskelldb-hsql-sqlite3-dev
  libghc6-haskelldb-hsql-sqlite3-dev
  libghc6-haskelldb-hsql-postgresql-dev
  libghc6-haskelldb-hsql-postgresql-dev
  libghc6-haskelldb-hsql-odbc-dev      
  libghc6-haskelldb-hsql-odbc-dev      
  libghc6-haskelldb-hsql-mysql-dev     
  libghc6-haskelldb-hsql-mysql-dev     
  libghc6-haskelldb-hsql-dev           
  libghc6-haskelldb-hsql-dev           
  libghc6-haskelldb-dynamic-dev        
  libghc6-haskelldb-dynamic-dev        
  libghc6-haskelldb-dev                
  libghc6-haskelldb-dev                
  libghc6-haskell-src-dev              
  libghc6-haskell-src-dev              
  libghc6-harp-dev                     
  libghc6-harp-dev                     
  libghc6-happs-util-dev               
  libghc6-happs-util-dev               
  libghc6-happs-state-dev              
  libghc6-happs-state-dev              
  libghc6-happs-server-dev             
  libghc6-happs-server-dev             
  libghc6-happs-ixset-dev              
  libghc6-happs-ixset-dev              
  libghc6-happs-data-dev               
  libghc6-happs-data-dev
  libghc6-gtkglext-dev
  libghc6-gtkglext-dev
  libghc6-gtk-dev
  libghc6-gtk-dev
  libghc6-gstreamer-dev
  libghc6-gstreamer-dev
  libghc6-gnomevfs-dev
  libghc6-gnomevfs-dev
  libghc6-glut-dev
  libghc6-glut-dev
  libghc6-glib-dev
  libghc6-glib-dev
  libghc6-glade-dev
  libghc6-glade-dev
  libghc6-gconf-dev
  libghc6-gconf-dev
  libghc6-ftphs-dev
  libghc6-ftphs-dev
  libghc6-fgl-dev
  libghc6-fgl-dev
  libghc6-edison-core-dev
  libghc6-edison-core-dev
  libghc6-edison-api-dev
  libghc6-edison-api-dev
  libghc6-configfile-dev
  libghc6-configfile-dev
  libghc6-cgi-dev
  libghc6-cgi-dev
  libghc6-cairo-dev
  libghc6-cairo-dev
  libghc6-binary-dev
  libghc6-binary-dev
  libghc6-arrows-dev
  libghc6-arrows-dev
  libghc6-anydbm-dev
  libghc6-anydbm-dev
  libghc6-alut-dev
  libghc6-alut-dev
  hmake
 |hmake
 |haskell-utils
 |haskell-mode
 |haskell-doc
  haskell-devscripts
  ghc6-prof
  geordi
  drift
 |cpphs
  c2hs