Every cynic is a hopeless romantic
One possibility is that your MC doesn't 'change', so much as the circumstances change and we see another side of her.
For instance, we meet her at her normal status quo which shows us her hard-rule barriers and boundaries. It doesn't come across as vulnerable or defensive because it looks like the actions of a tough person who hates sentimentality and weakness.
But circumstances change her 'mode' from defensive to protective, or infatuated, or just being invested in another person – a 'weakness' she was defensive to avoid.
This is a setup for putting this character in jeopardy. There is a reason she avoids this mode. It's dangerous to her well-being, and she's experienced this vulnerability before with bad results.
Avoid the cliché
Read this answer before commiting to this character turn.
In Samuel R. Delany's "Characters" essay, he relates his (anthology editor) wife's criticism that female characters usually fall into 2 categories: 'simps' and 'bitches', and the worst cliché of all: a bitch who becomes a simp after meeting the hero.
The problem isn't the character's turn, but that the character is just a prop – a strawman antagonist for the hero to easily conquer – not an actual character with their own motives and consistent values.
Does anyone know of characters from books & films or real-life females
who were tough and then softened?
All of them...?
Rarer to find a 'tough female character' who never goes through this arc.