I found soution for you as follows:
df2 = pd.read_csv('et_users.csv', header=None, names=names2, chunksize=100000)
for chunk in df2:
chunk['ID'] = chunk.ID.map(rep.set_index('member_id')['panel_mm_id'])
df2 = pd.concat(chunks, ignore_index=True)
Please note here you need to append each chunk to a list and after that you need to use concat to concatenate them all, also the ignore_index is not necessary.