Logistics of transferring domain and subdomains from Wix

I'm in the process of leaving Wix and I'm currently creating a whole new website on a temporary domain with inmotion. My plan is that once I've got the new website exactly how I want it, I'll point my domain here and then transfer it.

I'm concerned about the logistics of keeping things running during this process. I use FreshLearn as a subdomain on my website. So students access their classes using classes.mydomainname.com. (Only with my actual domain name.) What's the best way of making sure they are able to still access everything? Would the subdomain work on Wix or on inmotion or neither when I'm pointing my domain to inmotion? Will it stop working when the domain transfers? What's the best way of setting it all up? I'm thinking that when I set up the domain to point to inmotion, the subdomain will still be associated with Wix? Will I just have to watch for when the transfer goes through and set up the subdomain again?

If necessary, I can give everyone a direct link to use during the transfer process but I'd prefer not to because people get so confused when I change anything. 

Thanks for any advice!