I'd be happy to help out.
Is there a concrete plan?
Not a very concrete one. Someone needs to do some planning, too. IMHO, we should set up Redmine at OSUOSL and make a transition to that almost immediately, because the conversion can be pretty much automatic.
Has the decision to move to Redmine been made (already)? Last time some were arguing Trac might be 'fine'.
That will require some work on the Redmine end, I expect. OSUOSL can probably help us with some of the Redmine database rewrites that will be required, but I am not sure of that.
At that point we should probably also give each library its own Redmine project, or, if we are modularized at that point, possibly give library maintainers the option to use the GitHub issue tracker associated with the library's repository there.
Why not move to a GH tracker right away? Olaf