Hopefully someone can help me…
If I wanted to replace a workout in my Custom AT plan which I have in my calendar with a completely different type of workout would this screw up the adaptions going forward or break the AT plan?
I’m thinking if I don’t do the Saturday Threshold workout but instead do a 2 hour Zone 2 endurance ride and delete the Saturday Threshold workout, would this screw everything up?
Thanks
You can do it. AT may adapt other workouts, but that is what it was designed to do.
I can help, too! @rkoswald is on-track, but I just want to confirm:
By this do you mean, a plan made via Plan Builder that has been adapted by AT?
Looking at your career page, I’m pretty sure thats what you mean, but just want to be sure, in which case: YES. You can delete/drag/rearrange/TrainNow/substitute workouts within your plan, and Adaptive Training will absolutely take that into consideration when targeting the systems you need to work on at that specific time. 
Let me know if you have further questions!
Super thanks.