I have not done a handlebar swap, but I know the first advice members will share with you is to purchase a Genuine Harley-Davidson Service/Shop Manual so you have detailed directions, torque specs, etc. when working on your bike.

Congratulations on your new bike by the way!

 

If you go to a low handlebar on the 2020 FXBB, you will need a shorter upper brake line and also a shorter upper clutch line, if your cables are currently OEM length. You will want to pay attention to your new brake line banjo angle to see that it is correct for the new handlebar you picked. Hope this helps!

 

It depends on how much shorter or longer the handlebars are.
Measure from the center of the bars between the riser centers to the end of the OEM bars then compare that to the same measurement on the bars getting installed. In most cases 3" on either end won't require changing wire harness or cable lengths with some minimal rerouting to those.
The front brake line is the tiebreaker, longer bars will require a longer brake line. Shorter bars (<3") and the brake line can be rerouted.

 

I don't like the higher bars either and mine had been replaced to even higher ones when I bought the bike as an insurance claim so I bought another set off Craigslist and I think they are Heritage soft tail bars. THen I had to buy new bar mounts which I also got off Craigslist. I've never done a swap before either but got my 25 yrs old son to help hold them up while I we swapped out controls. Cables might be a little long but the bars are wider so I think that equaled out. Pictures of current bars. I've also changed tank to a Fat Boy.