This is tough! I struggled with this for years in L&D. Just getting time allotted in the plan for training to be developed and happen is so hard! My best advice is to get as involved as you can in the project to understand everything happening. Then, build your training plan and ask to have it integrated into the larger project plan. Once they see you need 2 weeks (just an example) to build the training materials and then another week (again, just example) to get all of the end users through a training session, with at least 1 make up session the following week for anyone who missed or was on vacation, hopefully they will have a deeper understanding.
Another suggestion is if you're implementing a new system through Agile methodology, build the training with the sprints that are happening. You will likely have to make changes as decisions are made, but you'll be in a much better place than if you wait until the system is fully designed to start building your training. I also try to push for a pilot group and to have actual users be part of the testing of the system.
That's just the training piece! There's so much more to managing a change.