Hi there, @tharun_p 
Thanks for reaching out! We are more than happy to help here.
You will have access to the amount of bandwidth you pay for. You can view the breakdown of our monthly bandwidth in MB per tier on our pricing page. Additionally, for a breakdown of how we track our bandwidth, please refer to the billing section of the Netlify Docs.
To answer your hypothetical, if you immediately upgrade account levels to have access to a feature and then immediately downgrade, you will not retain access to said feature. In order to maintain access to the higher bandwidth, I would suggest staying active on the target billing plan for the duration of the billing cycle and then downgrading when you no longer require the feature. That being said, we most certainly want to make sure you feel empowered to use the starter level bandwidth strategically and efficiently. We wrote the following Support Guide for this exact reason:
Take a look at that guide, as well as the pricing page and docs that I linked above. If you have any remaining questions, don’t hesitate to reach out!
Best,
Hillary