This is not equivalent. You could clear() and then append, but for the sake of clarity I would actually prefer something along the lines of:

valuueCopy->aggregateIds() = [&] {
QVector<QByteArray> v;
v.reserve(result.aggregateIds.size());
std::transform(...)
return v;
}();

While this is slightly longer due to the temp variable, it makes clear that this is an assignment, and is probably similary efficient due to the implicit sharing of QVector, which makes assignment a very cheap operation (doesn't copy the data, but only the pointer).

Wrap it in a lambda please.