Requirement for import of bugzilla data: http://bugzilla.wikimedia.org/1 will need to become T1. Renumberings like adding 100 000 would also be acceptable, as last resort (bug 1 -> T100001, bug 323 -> T100323 etc.).
It wasn't spelt out anywhere, sorry if it was considered obvious.
There are well over 15,000 links to specific bugs on Bugzilla at the English WIkipedia alone. There are thousands more at other projects, not counting links in Gerritt and code. Are all of those links going to break?
Notice that my definition of "break" means that if you click on [[bugzilla:12345]] or https://bugzilla.wikimedia.org/show_bug.cgi?id=12345 or any similar link, that you end up at any page other than the exact bug that the link originally pointed to (in this instance, a WONTFIXed bug titled "Server should return a 410 HTTP status code for deleted pages"). Going to the main page of Phabricator is "broken". Going to some other bug is explicitly defined as "very, very, very badly broken". The only outcome that is defined here as "not broken" is the one in which I click on an old link and it takes me to exactly the same bug report (only located in Phabricator) that clicking on the link would take me to in Bugzilla.
Note that all I care about is the links working, because all I use is the links. If all the old links will work perfectly, then my own immediate needs will be met. Other people may care about the numbers themselves, e.g., if they encounter the numbers in non-link form (a comment on a line of code, for example).