Yes, changing the blueprint won’t change what has been built. This is the current design.
I don’t think it’s a bad design, as you’d not want to update completed work as that would confuse how it has been done.
And task templates.
A workaround
A way to shorten the feedback loop would be to split the template into separate phases. Let’s say start, middle, and end.
The time to setup would increase as you’d have to make a new project for every project phase, but all changes made to any of phase templates are applied to all projects preparing to start this phase and using the template.
If you still feel this functionality needs improving I would suggest finding an existing feedback thread to add your vote and voice, or creating a new one if it doesn’t exist. See: 📚 How to share your product feedback