Players who have stockpiled Classic honor will have a huge advantage the moment they ding 70 in TBC, purchasing full PvP sets on day 1. Farming honor in a previous expansion should not let you purchase PvP gear in the new expansion. You should have to earn it fresh.
Don’t let the honor stay, otherwise you are going to have level 70s on day 1 with full PvP sets griefing and wiping the floor with players in quest greens.