Well, I lived that very tale, and I'm here to tell you that it's both doable and doable easily - for a price.
My first step was to email email@example.com. I laid out my dilemma - I was registered for the full but wanted to switch to Goofy - and requested information as to how I could make that switch.
A couple days later I received a very nice response telling me that it wouldn't be a problem provided that I took the following steps:
First, I was to register for Goofy outright. You do this like you would if you were starting fresh. Don't dilly-dally as if the race you want to transfer to sells out they can no longer help you.
Once this was done, I was to fill out a transfer form. This was a PDF attached to the initial email I received. It wasn't too complicated - it asked for my basic information plus the registration numbers of my former marathon registration and my new Goofy registration.
I sent this back in an email response and voila! Well, not entirely voila. I was assured that once the form was processed I would receive a refund on my original marathon registration, which I did - minus a $45 transfer fee. This strikes me as a little steep but I think we've all accepted by now that when it comes to pricing, Disney does what Disney wants.
And that's it! As far as I'm aware you can follow this process for transfers in any runDisney race weekend. There are a couple of things to keep in mind, however.
You can only transfer up, not down. The form I received (which I can only assume is customized per race weekend) offered the opportunity to transfer from the 10K to the half, half to full, full to Goofy, and Goofy to Dopey (with any combination leapfrogging to a higher distance permissible). There's no option to go backwards - if you're in the marathon, you can't get any money back dropping down to the half.
The other thing to note is that there is a transfer deadline. For this year's marathon weekend it's October 8, but it's going to be different every race weekend and year. If this is something you're pondering, make sure you check that deadline now so that you don't miss out.
runDisney mentions the ability to make transfers in your account, which may be a new feature. You can check that out on their race policies page. The above was my experience in April 2019 but things do change. The good news is you can always start by emailing that firstname.lastname@example.org email address - it's a pretty low risk way to start.
Hope this helps somebody! See ya at the Goofy races - I'll be the one who's just relieved she woke up on time. >D
Don't forget, you can follow FRoA on Twitter @fairestrunofall. If you have any questions, leave a comment or email email@example.com. See ya real soon!