My Underground Hallow didn't spawn

My Underground Hallow didn't spawn... Because my Hallow was changed from my old forest to my new Hallow so what should I do? I need souls of light and other things like cave crystals.

The generation of the Underground Hallow biome isn't a column underneath the Hallow - it's one half of a V shape starting from the centre of the map's Underworld, the other side of which is either Corruption or Crimson (depending on which of these was initially on the map).

The Terraria Wiki's Hardmode entry has an image showing an example of the V shape from the middle of one map - Hallow is on the left side of the V in this example.