وَأَنَّا لَمَسْنَا ٱلسَّمَآءَ فَوَجَدْنَـٰهَا مُلِئَتْ حَرَسًا شَدِيدًا وَشُهُبًا
Wa annaa lamasnas sa maaa'a fa wajadnaahaa muli'at harasan shadeedanw wa shuhubaa
And we reached toward the heaven and found it filled with mighty guards and blazing flames.
Testimony here is first-hand: the jinn are reporting what they met when they went up, and the verb lamasnā is the one for feeling your way toward something by hand. Verse 9 dates the change with the word now, so what they describe is a night watch that has just been reinforced. Whether the barring began then or was only tightened has long been argued, since 15:17 already describes the sky guarded.