In the end, you have to be true to yourself. Keep connections with your friends in the traditional church, if possible. If they decide they can't do that, that's regrettable, but they are more to be pitied than anything else, if they have become so dogmatic they can't be friends with someone who no longer believes in Christianity or certain doctrines. From my personal experience, traditional Christians never treated me that way (though they might still pray for my soul -- that's annoying, but no one's perfect, and I'm sure they mean no harm) -- more of the hostility came from fundamentalists or fringe sects.
I would be honest about wanting to explore other aspects of your spirituality, but how far you go into the details is up to you. One thing I've learned -- and this is something many people have not learned in places like the South where I live -- is that I don't have to tell anyone else what I believe or don't believe. It's none of their business unless I happen to enjoy the discussion.
Edit: By the way, there is no need to be an exclusive member of a Unitarian Universalist congregation even if you do decide you want to sign the book and become a member. In Unitarian Universalism, people are free to explore their spiritual path as they wish and do not have to revoke their membership in a prior religion or denomination to become a UU, nor are they prohibited from becoming a member of another spiritual path while remaining UU at the same time.