Sometimes word changes in old hymns/songs/carols to modernise language are helpful, making clearer the intent and removing obsolete idiom.  Sometimes they manage to lose, damage or even destroy something of the poetry/mystery/meaning the original writer captured/expressed.

BPW 145 begins thus:

Of the Father's heart begotten

When the worlds had yet to be

He is Alpha and Omega

He the source, the ending he,

Of the things that are, that have been,

And the future years shall see:

Evermore and evermore

The 'traditional words' version I learned years ago, and which is in BHB begins:

Of the Father's love begotten

Ere the worlds began to be...

And another even older version has:

Of the Father's heart begotten

Ere the world from chaos rose

He is alpha and omegea

He the source and he the close

Of whatever is or has been

or the future years disclose

Saeculorum secoulis

For me, somehow "ere the worlds began to be"  is richers than "when the worlds had yet to be"... something about 'ere' an archaic word that does, simply mean "before' echoes better the mystery of the eternal Christ who is beyond/outside time and space.

I am intrigued by the plural "worlds" in the majority of versions of this hymn, which has a clear 'universal' referent rather than "world" which preusmably means earth.  The hints of John 1 and John 3 are there for those who will see... in love God created all that is, in love for the cosmos, God entered its finitude to redeem it.

A beautiful hymn worth singing - in whichever version you favour!