Hola Ελισάβετ

words starting with -hie aren't changed if preceded by the conjunction "o". This change to "u" only affects nouns starting with -o or -ho. 

Words that start with hie- are not even affected by the change y to e (see lesson at the bottom) because the sound of "hie" is more of a "ye" sound, therefore we can write:

See how conjunction y changes to e

I hope this helped.

Saludos

Inma