I guess that would be an 'it depends'. I believe the recommended process is to connect the DCS system and try it before making any changes or doing any rewiring. It could work just fine as soon as you hook it up, or you may have to make some adjustments. If you need to make adjustments it could be as simple as adding a block or two here and there, a lighted bumper or something like that. IMO, it isn't that difficult, but I really started from scratch on my current so I didn't have existing wiring to contend with.
I would recommend one of Barry's books though, there is a TON of good info in those and after the initial purchase it will be referred to for many years to come. I have read the DCS Companion a couple of times and looked up countless items over the years. However the new DCS WiFi Companion just arrived yesterday and I have not gone through it to see what I think. It's supposed to be basically the same book only written for the new MTH app instead of the DCS remote. Maybe he will be along to clarify. He also provides a LOT of help to users here on the forum that have problems. He does this whether they have purchased his book or not.
Hope that helps a little. I am not an expert like some here, but I try to help where I can. Others here are much more knowledgeable. Either way, good luck with you choice here. Oh, one more thing, without DCS you are leaving a lot of NICE MTH engines on the table that you may really enjoy if you had them. I really like both MTH and Lionel and would hate to be stuck with only one of them. Again, Good Luck !!
I see Barry posted while I was typing. Of course I defer to his post over mine in case of conflicting info.