This situation (upgrading server during traffic spike) is an example of why saving a $20-50/mo on hosting isn't worth it, unless it's for something non-commercial.

If you've got maybe tens of thousands of dollars of time (and money?) invested in your app, what's another $40-80/mo for better hosting (e.g., 8GB/4 cores on Digital Ocean is $80/mo), in case something like this traffic spike happens? The app reaches the top of HN, where thousands of people that use Trello, Basecamp, and/or Freshbooks (I use all 3, every day) will be frequenting, and you've lost all that revenue.