So I'm playing a Witherbloom and the description is this:

You choose two cantrips from chill touch, druidcraft, and spare the dying, and can choose one 1st level Druid or Wizard spell. Once per long rest, you can cast the chosen 1st-level spell without a spell slot. You can also cast it using any spell slots you have. 

Am locked into that spell or can I change it with each long rest?

You are locked to the chosen spell.

The once per long rest part is only for casting the spell without using a spell slot.

That’s what I thought. Better choose carefully then.

Same goes with the cantrips, you choose two out of the three and you are stuck with them.