If you think that the things will change on 4th November if the Dems gain a majority in either House, you are wrong. The winners of local elections will not take up their seats until 3rd January 2027. Between November and February, each State has to confirm and certify the winners of each seat within it. This can takes weeks and you can be sure that both Parties will be calling for recounts in close seats. How close? How long is a piece of string in the case of teh Republicans.
After each State has confirmed the results in its seats, a certificate is sent to the Clerk of the House. Then in January, all those who have been elected congregate in the House they have been elected to and the Clerk Clerk calls the roll, giving a seat to the person called. That's why that ruckus happened on the 6th January 2021. It could happen again.
Here's a video that give a description of what Trump and his cohorts could do if the Republicans cop a beating.