I've written a song using a poem from 1923 as the lyrics and my own new melody. What a novel idea! (Yeah, I've seen past threads about Byron and Tennyson et al). My questions:

What are the copyright considerations if I decide to record this song?
How do I get permission and/or license to use the poem as lyrics? Harry Fox agency seems to deal only with songs.
Do the poems eventually become public domain?
If I change a few words (to fit the song) does that make the lyrics mine?

I actually intend to give the poet credit, since this is an old favorite poem of mine which I want to acknowledge, and I want to do the right thing. Plus, most people will recognize the poem (or am I in English Major Oblivion?).