The change here assumed must have been pre-Italic, for the later combination -nv - remained, regardless of the origin of the v. Such forms as convenio, invītus are plainly late compounds, since the phonetic forms would have been * conguenio, *inquītus. That does not mean that the old compounds did not exist, but simply that there was a constant re-formation of con+venio. The same explanation would hold for words with original v, as in con-veho. Wherever nv come together in Latin or in the dialects, it follows either that there must have been a vowel between n and v or else that it was a new compound, that is, new as explained above.
A Wed, study studied this question.