Actually changing a word from definite to indefinite by adding ال doesn't change the short vowel mark (al-ḥarakah الحركة) on the last letter.
Only one thing might change:
If the word ends with tanwin تنوين (nunation -un, -in, -an) (the addition of a final nūn sound to a noun or adjective to indicate that it is fully declinable and syntactically unmarked for definiteness), then tanwin changes to the convenient ḥarakah when adding ال:
-un ٌ becomes ḍammah ضمة (a short /u/)
-in ٍ becomes kasrah كسرة (a short /i/)
-an ً becomes fatḥah فتحة (a short /a/)
But no need to worry about that, because in colloquial Arabic we don't pronounce al-ḥarakah on the last letter. So we pronounce all words as if al-ḥarakah above the last letter is sukūn سكون (this ḥarakah indicates that the consonant to which it is attached is not followed by a vowel).