This happens in the episode where Tai finds his crest and Etemon shows up for the first time. Shortly before Etemon attacks the kids, Agumon turns into Greymon and get rid of a couple bunny-like digimon. (I don't know how to spell their name but that's not important.)

Anyway, etemon shows up and starts destroying the Koromon village. Shortly after one of the other digimon digivolves, we see a shot of the kids. Right in front of them, agumon is standing. However, a copule of shots later, when etemon attacks the digimon, it shows a shot of greymon getting turned back into agumon.