After a lot of tests, I tried to do the upgrade on my production environment... but fails 'cause can't import the environments:
tmp]# ll /tmp/chef11-server-data20141209-13680-18oxyxs/
total 115k
drwx--S--- 8 root root 4.1k Dec 9 18:07 ./
drwsrwsrwt. 15 root root 4.1k Dec 9 18:57 ../
drwxr-sr-x 2 root root 21k Dec 9 18:06 clients/
drwxr-sr-x 318 root root 13k Dec 9 18:07 cookbooks/
drwxr-sr-x 25 root root 4.1k Dec 9 18:06 data_bags/
-rw-r--r-- 1 root root 24k Dec 9 18:07 key_dump.json
drwxr-sr-x 2 root root 25k Dec 9 18:06 nodes/
drwxr-sr-x 2 root root 13k Dec 9 18:06 roles/
drwxr-sr-x 2 root root 4.1k Dec 9 18:06 users/
...
Pulling open source Chef 11 database credentials
Finished downloading data from the open source Chef 11 server
Ensuring open source Chef 11 server is stopped
ok: down: bookshelf: 0s, normally up
ok: down: chef-expander: 1s, normally up
ok: down: chef-server-webui: 0s, normally up
ok: down: chef-solr: 0s, normally up
ok: down: erchef: 1s, normally up
ok: down: nginx: 0s, normally up
ok: down: postgresql: 1s, normally up
ok: down: rabbitmq: 0s, normally up
Open source Chef 11 server data downloaded to chef11-server-data20141209-13680-18oxyxs
Created /tmp/chef12-server-data20141209-14126-113ym99 as the location to save the tranformed data
Transforming open source Chef 11 server data for upload to Chef 12 server
/opt/opscode/embedded/lib/ruby/1.9.1/fileutils.rb:1515:in `stat': No such file or directory - /tmp/chef11-server-data20141209-14126-4pi3rz/environments (Errno::ENOENT)
from /opt/opscode/embedded/lib/ruby/1.9.1/fileutils.rb:1515:in `block in fu_each_src_dest'
from /opt/opscode/embedded/lib/ruby/1.9.1/fileutils.rb:1531:in `fu_each_src_dest0'
from /opt/opscode/embedded/lib/ruby/1.9.1/fileutils.rb:1513:in `fu_each_src_dest'
from /opt/opscode/embedded/lib/ruby/1.9.1/fileutils.rb:436:in `cp_r'
from (eval):121:in `block in transform_chef11_data'
from (eval):120:in `each'
from (eval):120:in `transform_chef11_data'
from (eval):45:in `run_upgrade'
from (eval):135:in `block (2 levels) in load_files'
from /opt/opscode/embedded/lib/ruby/gems/1.9.1/gems/omnibus-ctl-0.3.1/lib/omnibus-ctl.rb:177:in `call'
from /opt/opscode/embedded/lib/ruby/gems/1.9.1/gems/omnibus-ctl-0.3.1/lib/omnibus-ctl.rb:177:in `block in add_command_under_category'
from /opt/opscode/embedded/lib/ruby/gems/1.9.1/gems/omnibus-ctl-0.3.1/lib/omnibus-ctl.rb:555:in `run'
from /opt/opscode/embedded/lib/ruby/gems/1.9.1/gems/omnibus-ctl-0.3.1/bin/omnibus-ctl:31:in `<top (required)>'
from /opt/opscode/embedded/bin/omnibus-ctl:23:in `load'
from /opt/opscode/embedded/bin/omnibus-ctl:23:in `<main>'