I just finished an A race event - one I had been training for since the beginning of the year. My next A race event is in September. I’ve created another training plan, but it looks like it’s designed to continue to grow my fitness/FTP. How can I modify the plan to maintain my current fitness/FTP vs. growing it? I will not have as much time to dedicate to training these next few months as I did earlier this year, but I at least want to keep what I’ve gained.
Welcome to the forum! 
I’m not 100% sure that I follow.
What would a plan designed to keep your FTP where it is look like in comparison to one where your FTP might grow?
I’d recommend simply building your next plan with the availability that works best for you and carrying on.
Focusing on keeping your FTP as is rather than growing it seems like a unique and specific goal. Instead, I’d recommend just training when you can and hoping for the best.
Let me know if I’m missing something here.
Is this a really unusual and unique goal? 
I can understand the OP’s inquiry. Even the plan builder has a “maintain fitness” setting. Maybe you just want to keep the intensity high enough not to lose fitness but not so high that it taxes your nervous system enough to make other areas of life more challenging?
I suppose you could achieve something like this by manually lowering your FTP but then won’t the AI just keep pushing you upwards?
A workaround for the OP would just be to setup a normal plan and keep an eye on the FTP prediction. Skip workouts as needed to keep the prediction more or less steady.
I always just recommend building a plan that fits in with your availability and goals and going for it.
If you’re feeling fatigued or stressed, it’s okay to pull out some intensity or volume. Masters plans work well for that.
We do have plans for fitness maintenance, but you’d still need to balance intensity and volume in the same way. There’s nothing magic about those plans that necessarily make them less demanding, I wouldn’t say.
Here’s our official description for the maintenance plans:
Not every cyclist is targeting specific events, and sometimes your goal isn’t so much to build fitness as it is to maintain what you already have. If this describes you, consider the Maintenance specialty plans. These interchangeable 2-part blocks incorporate a mix of high-intensity workouts to keep you sharp, with added endurance workouts in the mid and high-volume versions to maintain base fitness. These plans are challenging by design, but they’re a truly effective and rewarding way to stay fit and fast.
There’s been times I’ve felt like the OP. I want to keep riding and stay fit, but I don’t want to push myself as hard as TrainerRoad wants to push me. I think dropping volume in that case Plan Builder just increases intensity. Perhaps changing from a Balanced approach to Conservative is the thing to do?
Training Approach could help some, but we aren’t going to stuff you full of intensity if you reduce your volume.
You can build whatever plan you’d like in Plan Builder. Two hard days is probably a good place to start if you’re looking to “take it easy.” 