Dear Raga
Certainly am not mocking any devotees. If you are very strict that only hari namavali gives moksha, wont the namavali of subramanya (Shri swaminatha swami) wont gives that??? Initially itself, I apologise and then only asked that, it is mainly to note the seriousness of the issue. There are lot of differences between kuladevatha and isthadevatha. You might be having n number of ishtadevatha or even one. (In your case "perumal", but certainly your kuladevatha is the one who has originally with your forefathers' worship (swaminatha). Am not descriminating any gods or goddess. Infact my kuladevatha is "ayyanar" in a small village amidst of farmlands. But I love goddess "durga" and even changed my name as "durgadasan". But certianly my kuladeivam is that ayyanar only. Tell me how can I change that as I wish. Then the whole concept of kuladevatha itself will die.
See in our community, a sampradaya of "Samaradhana" will be done for Kuladevatha. It is very famous for people having SWAMINATHASWAMI. I know well that. If you keep on changing this to some other god, what will be the situation/rights to this particular kuladevatha then???
Finally after few generations, our successors will be wrongly guided and they have to seek the help of some astrologer or mandhrik to search their original kuladevatha for remedial measures. But am damn sure that your kuladevatha will bless even under that condition, since it is like your own parents. Whether you part them or ill treat them, they live for you. That is the nature of kuladevatha also.
Why am saying this much intensed about this is, two of my friends who lost their kuladevatha has lost their lives itself. Am not saying this just as story sake.
Finally, even in your post, what you have requested, "If there are others who've done so ie adopted another diety as their kula-devatha , they can share their experiences in this thread"
Am sure that you know the difference between adopting and parenting...
Pranams
Once again am sorry if my post hurts you in anyway. But please keep your deity with yourself. Atleast dont pass that to your next generation. Show them and teach them your actual kuladevatha.