If dishes have only ever been used for kosher food but they were not toveled (immersed), what steps must be taken per halacha to be able to continue using them? Can one just take them to the mikvah now, or do you have to treat them as if they had been used for non-kosher food and start over (if possible) with boiling, heat, etc to kasher them first?
Only kelim (vessels) which absorbed issur (forbidden substances) need to be kashered. A kli which may have issur stuck to the surface, but not absorbed should be scrubbed. Keilim which were only used with kosher are clear to be toiveled. See Shulchan Aruch, Yoreh De'ah 121.
Another point concerning old keilim is to make sure there is nothing on the surface which would be a separation between the kli and the water, see the previous siman YD 120, siff 13 concerning rust on a kli.