You can see a quick breakdown of the Starter/Free plan comparison/differences here:
https://answers.netlify.com/t/is-the-old-starter-plan-technically-the-same-as-the-new-free-plan/133543/2
Yes, although you may need to contact support to do so.
The page will advise you when you try.
Yes, you won’t ever be charged on the ‘Free’ plan.
You don’t even need to have billing details against the account.
Yes, your site, (and even your DNS if using Netlify DNS), will be disabled until the end of the billing period.
This will occur if you exceed any of the plan limits.
There is no ‘Basic tier’, so I presume you mean the ‘Starter’ plan.
Nothing will need to be changed with your DNS.