Disclosure forms can be amended retroactively. It is done all the time.
In fact, the meeting in question was "discovered" as the result of Jared Kushner's amending his disclosure form.
Of note; according to the news reports there were three other people present at the Trump Jr. meeting.
This included: Rinat Akhmetshin a Russian-American lobbyist; a translator; and a representative of the Russian family who had asked Goldstone to set up the meeting.
The "Russian Agent" Trump Jr. is supposed to have failed to mention is Rinat Akhmetshin...who happens to be a naturalized AMERICAN citizen.
https://en.wikipedia.org/wiki/Rinat_Akhmetshin#cite_note-Eckel-7
Nor is he registered as a foreign agent under the Foreign Agents Registration Act (FARA), although some believe that he should be.
He is a lobbyist for hire and has been working with Veselnitskaya's organization to overturn the Magnitsky adoption ban sanctions.
As he is an American citizen, there is no obligation to report him as a "foreign contact" on any disclosure form.