Nalanku or nalunku is, to my knowledge, a device to create some basic camaraderie between the newly wed couple, and can be said to have served a purpose in the days of "arranged" marriages. Even so, when I got married in 1964, we (myself & my wife) agreed that we need not have any nalunku, or the groom being paraded around etc., because we thought that as we knew each other already as small kids, all these extra items could be dispensed with.
Among our people (tabras from TNLY district) there was no custom of changing the original thaali or thaalikkayiru, 3 months after marriage etc.