Wouldn’t they all be based on each individual purchase date? I.e. anything purchased Before 01/01/2021 would be a long capital gains tax versus a short?
Rock if you go into your setting/preferences in your brokerage there should be elections for how you want your share purchases/sales handled. For example “First in-First out”. This election would be tax advantageous as your longest held shares would be the first to be sold if you decide to sell, therefore exposing you to the lower long-term capital gains as much as possible.
You can choose which shares you want to sell when that time Comes. You should have an option called (select lots) and you can pick which block of shares you want to sell and will have the date purchased next to it.
The default is FIFO - First In, First Out. However, you can specify whether you want FIFO or LIFO - Last In, First Out. I believe that PNW Ironman is correct that you would want to sell shares held longer than one year to get the tax rate on long-term capital gains.