My understanding is that it is preferable to kasher oven racks used in a conventional oven for Pesach with libbun gammur especially if on a few occassions bread (e.g. pittot) were heated directly on them.
What is the implication after the fact if one used such oven racks and they were only kashered for Pesach by the method of libbun kal or haggala, rather than libbun gammur?

See mishna Brurah 451:32 allows it only with a Hefsed Merubah and if it wasn't used within 24 hours with Chametz. in ibid 160 he seems to be more Meikul and permits Hefsed Miruba or is not a Ben Yoma (used within 24 hours.

In a nutshell, the Sugya revolves around whether Chametz is considered Heter Bala (absorbed while still Heter) which only needs Hagala (or Liban Kal, which is no worse than hagalah,) since before Pesach, Chametz is still Permitted. Or do we say that since it's always Chametz, it's always has the status of Issur on it, since this Chametz is always forbidden on Pesach. This is not compared to dairy or meat by itself that are actually always permitted, or Kodshim before it becomes Nosar.

Also, we through into the mix the Shaila whether Nosan Tam L'Fgam is forbidden, since we Assur even the smallest amount and doesn't need to give Taam. Or do we say that they weren't Gozer on Bliyos that would never give Taam and they're not even Ussur.

http://limudtorah.jewishweb.org

http://globalyeshiva.com/eve/forums?a=frm&f=9291076782