Under current rules, Convention Delegates have one shot to nominate: at the convention. If the nominee were to step aside or die, or for some reason the convention were to adjourn without nominating someone, the nominating authority devolves upon the DNC.
However, the delayed-acceptance scheme will require a whole new set of rules to be adopted. The new rules would either permit the nomination to be made, but not accepted, for a time certain, or to permit the Convention to recess, but not adjourn, and reconvene in September actually to nominate Kerry.
The overlay on all of this is that the FEC would have to go along; its rules, also, don't really accomodate a delayed acceptance. Under the current FEC rules, a convention is defined by the nomination; either Kerry will be nominated at the Convention, or it won't, in fact, be a convention at all.
As the Democrats have demonstrated time and again, rules are for Republicans.