Of course they want to sell as much as possible, I have florabase in my 50g under all my rooted plants and carpet in the middle. And a large gravel and rock section on the right and sand on the left of the tank. I just used hardscape wood and rocks etc to keep the substrates divided.
I was in the same boat as you, I decided to add a carpet after I had planted so I did the following. I run two filters on my tanks one on each side, I started taking plants out on the right hand side and all the fish bolted for the left. I then put in a lexan divider and left both filters running. I stared to pull out the unwanted substrate and add florabase and replanted and pulled the divider. I then switched the filters around so the dirty one that had been sucking up all the cloudy mess from my substate change to the left of the tank and started to pull the plants on the left. All the fish will bolt to the right put in the divider and do your work on the left. Clean the left filter up and pull the divider and all is well. Just make sure you do a large
water change after doing a large substrate job as it can cause algae outbreaks if you dont.
If your tank is planted heavy enough, you would not even notice a mini cycle even if you got one as the plants are very efficient as sucking up excess toxins. Especially if you have lots of fast growing plants in your tank. When ever I do a very large job on my tanks I always add some hornwort to my tank to suck up nutrients, I leave em for a few weeks and then turf em back into a plant nursery tank after they have done there job.