One of my friend is a Vaishnavite Brahmin....but he says, has increased devotion to Lord Shiva and is much affected by Shaivism. Since he is the eldest son in home, he became responsible for pujas, havans, etc which are laid on him as responsibility. As he has became a devotee of Lord Shiva, he tried to replace the family devotee as Lord Krishna to Lord Shiva, which was opposed by his family and his caste/sub-caste society. He asked for help. He want to convert to Shaivism officially/religiously, so that his people accept this. He said he doesn't want to fight with society, but want to live with peace. What he must do? He needs help????
Being a devotee of Lord Shiva does not give him rights to alter the Kula Devata of his family.So that is not right.
Moreover, if one is a "true" devotee of any God,then he sees all Gods as same and one with his Ista Devata.
He can worship Lord Shiva at the beginning of his daily religous duties before starting the pujas and havans that he is supposed to do for his family or community.
I can also not understand how by worshipping Lord Krishna will lessen the devotion that he has for Lord Shiva?
Lord Krishna Himself was a earnest devotee of Lord Shiva in his lifetime.
So i don't see why and how this is such a big problem for your friend.
You are mixing the two concepts of ishtadetaa and kula devtaa.
he can change his ishtdevtaa(due to "his" own freedom of religious sect), it is his individual choice.
but he can not change his kuldevtaa , just due to his own freedom of religious sect. For that all the persons in his kula/extended kula have to agree(democratically agree), as they all have their own freedom of religious sect. Freedom of choosing of choosing "kuladevtaa" is made of sum of agreement of every individual in kula right to choose kuladevtaa. And hence kuladevtaa does not change so often.
Why he needs to force his choice of ishtdevtaa as kuldevtaa on everyone? So his right to worship does "not" mean "only my right of worship". Right of worship means "right of worship by each individual(sihtdevtaa) and also democratic right of worship by each social group(say,kula)".
And hence he can personally worship the kuldevtaa(due to overall democratic majoritarian right of kula) "and" additionally he can also worship ishtdevtaa(due to his democratic right to worship). And everybody else can worship their own ishtdevtaa and the kuldevtaa. Thats the meaning of democratic right of worship.