It is unlikely any changes made and then approved

It is unlikely any changes made and then approved by congress would be approved retroactively for the 2021 tax year. That being said I will be selling long term shares and I am not too worried about whatever happens this next tax year.

When could Biden's changes happen and when is it most likely?
Here's an important point to know. The time when Biden's proposed tax changes could happen and when they're likely to actually take place are two different things.

We'll start with the earliest they could be implemented. Although Biden isn't being inaugurated until a few weeks into 2021 and any tax legislation would likely take several months at a minimum, it's entirely possible that changes could be made retroactive to January 1, 2021. If Biden and Congress agree to raise tax rates on the wealthiest Americans for the 2021 tax year, it's not completely out of the question.

Having said that, the earliest any major tax changes are likely to happen would be for the 2022 tax year, the first full year Biden is in office. This was the case with the Tax Cuts and Jobs Act, also known as the Trump tax cuts. Although the legislation was passed toward the end of 2017 and Trump had been in office since mid-January of that year, most of the changes made didn't go into effect until 2018. In fact, the only major point that was retroactive to 2017 was a relatively minor change to the medical expenses tax deduction.