The vowels are separated into three categories based on openness.

Closed:        i, ų, u
Semi-open:  ę, į, o
Open:          e, a, ą

The vowels harmonizes themselves with the first vowel in the word based on the openness.
When it harmonizes it adjusts to the respective letter of that category.

>Changes to Closed<
"e and ę" change to "i"
"a and į" change to "ų"
"ą and o" change to "u"

>Changes to Semi-Open<
"i and e" change to "ę"
"a and ų" change to "į"
"ą and u" change to "o"

>Changes to Open<
"i and ę" change to "e"
"ų and į" change to "a"
"u and o" change to "ą"

If Ǫǫ is implemented, it will be considered a wild card vowel.
It will fit in any of the above categories, thus it stays the same regardless of category.