Definitely pull your mods folder out before updating, installing the new pack and playing your game. Don't just copy it then play your game while hoping for the best outcome. This is a huge base game update and GT is introducing some features we have never experienced before. We have no idea what is going to be affected and playing with broken mods, CAS CC, and B/B CC can corrupt your game saves under certain conditions.
If you don't use mods or CC, you don't need to worry about what will happen to your game except for whatever new glitches and bugs that may come along for the ride. Since we won't see GT being played with other packs, we don't know if there will be compatibility issues to deal with. We just don't know what to expect until we play it. Bienchen83 posted a list of her mods she believes will be broken once the game is updated. Here is the link if you would like to read it. https://forums.thesims.com/en_US/discussion/953198/mods-by-bienchen83-sims4me-at-deichschaf-sims-alphabetical-list#latest. She isn't sure if there will be more broken ones. But she is quite certain the ones on her list will need to be updated.
Just play it safe and make a copy of your entire Sims4 folder. Especially if you created a new save or Sims for GT. It takes only a few minutes to do this, and it could save you from losing corrupted saves and starting over from scratch. Which is a real bummer. I have been rebuilding my entire game since my other laptop died and I lost everything. I'd hate to see anyone go through that when we have been looking forward to this update and EP for years.