The 5e Ecology of Night Hags does not include ritual transformation
From the 5e Monster Manual:
Monstrous Motherhood.
Hags propagate by
snatching and devouring human infants. After stealing
a baby from its cradle or its mother's womb, the
hag consumes the poor child. A week later, the hag
gives birth to a daughter who looks human until her
thirteenth birthday, whereupon the child transforms into
the spitting image of her hag mother.
Hags sometimes raise the daughters they spawn,
creating covens. A hag might also return the child to its
grieving parents, only to watch from the shadows as the
child grows up to become a horror.
The Ecology of Night Hags in previous editions included ritual.
For example, an article expounding on it in Dragon Magazine 324
Night hags reproduce in a manner exceedingly foul to mortal minds...
At the end of this period, the night hag gives birth to a dark-haired female child otherwise indistinguishable from others of her mate's species. Having no concept of maternal instincts, night hags always foster their children...
At any time between the child's first birthday and puberty, a night hag might return to perform a series of despoiling rites that cluminates in the child's transformation into a normal night hag.
The process begins with an initial visition...
After this initial ritual, the night hag must return three times, each visit thirteen days after the last.
On these visits, the night hag must suckle the child and feed it the flesh of a living larva, a process that takes an hour.
Here's the take on your question specifically:
If any of these feedings are interrupted, or if the night hag can't access the child by the end of the proper day, the child cannot be transformed into a night hag.
A similar yet abbreviated description of the reproductive cycle is in the 3.5 MM.
