Migrating the code from Launchpad (bzr) to GitHub (git) is not the big problem. But is there a way to migrate the bugs/tickets from a Launchpad project to a GitHub one?
There is a project called LP2GH ("Launchpad to GitHub"), which can do that. The link above leads you to a documentation about moving your issues.
This document will give you a detailed account of how to transfer your bugs, blueprints and milestones from Launchpad to GitHub and explain what to do once you’ve moved.
There is another project called github-issue-importer, which is aimed exactly at this task. It seems easier to use then the first one.
Github Issue Importer imports bugs into github issues. Currently it only supports Launchpad. It is hoped it will support other bug trackers in the future.
The importer will import each bug and it's comments into github.