Howdy Ed....
Well....I'd say no hard & fast rules.
I have key changes in many of my songs which I enjoy employing.
But...you could also just take the song up 1/2 step at that last verse to achieve a musical change, so to speak, which may work also.
Just a thought.
As an example of how it would sound here's a link to a play-along copy tune (rather than an original) I recorded many months ago.
On the last verse she (Randy Crawford) goes up 1/2 step (Cm - C#m):
https://www.youtube.com/watch?v=EnUlZIKArRE&ab_channel=chulaivetHope that helps a bit....