I don’t think Shriver had any problem with ballot access when he replaced Eggleston.(I could be wrong, it was 44 years !) However this is the top of ticket could be interesting, but as long she is breathing and conscious she will be on the ticket.
In that case I think it was not long after the Dem convention that the uproar over Eagleton's mental health status erupted. It turned out he had had shock treatments, back in the day.
Iirc, at first McGovern said he backed Eagleton "110%." Then shortly after that McGovern abruptly did a 180,º and Sargent Shriver was brought on to replaced Eagleton. This seemed to be ok with most Dems ... because for them, Sargent Shriver, being married to Eunice Kennedy, had that special Kennedy magic that used to allow Kennedys to get away with anything ... kind of like Clintons get away with everything today.
Anyway correct me if I'm wrong, but I think all this happened long before it was too late to legally change names on the November ballot.