You've got to. I wanted to run the adventure as close to true as I could, but customization is unavoidable.
I was tempted to make everyone start at level 1 with new characters just so we could walk through all the introductory missions, starting fresh seems like the only way to really use the book completely as-is. That's obviously why Red Larch is so well mapped-out, it's a mini-setting. But we all had 3rd or 4th level characters who had just completed the starter set LMOP (which was great, our current Halfling Fighter was the DM that time and I really enjoyed playing a forest gnome ranger) and tying the two adventures together is just so seamless...
I'm trying to insert our storyline into the book without disrupting too much, I think the book is great. The only things I find myself changing are the bad guys. The characters started out a little high-level for the stock "cultists" and "bandits", but I'm trying to challenge them more with tactics than numbers. The Crushing Wave Bandits attacked their caravan as a campaign opener, and I had them act like a bunch of sailors who have no idea how to successfully raid anything that isn't floating, to ensure an easy encounter. The Howling Hatred Cultists have developed multiple strategies for tossing people off the roof and have managed to do it four times already (Feathergale spire is turning out to be a lot of fun) and for some reason the "boss" bad guys are all real good at getting away.
I had read that a lot of players did not take the "Delegation" bait, and that it was a weak hook. I'm glad I read that before I started the campaign, it gave me time to think of a way to make the hook sharper.
I had the characters run in to the delegation in Triboar, before arriving at Red Larch. The party's Dwarf ended up spending an evening drinking with the delegation's Dwarves and made friends. They planned to meet up in Red Larch, and when the party got there it did not take the player long to ask "hey, are those Dwarves I met in Triboar around? They said they'd be here." I didn't even have to push, he started looking for them on his own. Simply drop a rumor about freshly dug graves, and he's off to Feathergale Spire...
The stolen books were there for him to find as well, and if they had simply gone to the tavern across the street instead they would no doubt be at Riverguard Keep rather than the Spire right now. My guys are going through it like a railroad so far, and it's not my fault or intention. They just keep making extremely logical decisions, methodically finding and following leads, and the story tells itself...
The fact that I can novelize our recaps is due to the fact that they actually do the things characters would do in a good fantasy novel, I don't have to roll my eyes while narrating some absurdity...