Its obvious that there will be absolute zero fixes happening on Old Wotlk core except maybe some critical fixes.
So my suggestion is before you launch Lordaeron , completely wipe Wotlk Bugreports in bugtracker to avoid confusion etc.I personaly visit bugtracker alot and when realm launches id like to browse trough bugtracker and not think "is this report from old or new core".
Lordaeron has already a separate slot in the bugtracker so there shouldnt be any confusion.
Imo it makes more sense to wait until the lordaeron core is implemented on the other wotlk realms and then wipe the old reports.
To be honest i think Warmane is gonna sort it out somehow like save the old reports but not actually wipe them off the bug tracker... Perhaps just remove them and keep the new reports just in one section.... Lord has a specific one at the moment but old reports before that category was implemented was usually marked [Lordaeron] on their bug reports.