I have had much success using the Keap API with Zapier to update contacts. This is for changes not natively supported in Zapier, e.g. I change the contact type from lead to client, add an owner, etc.
I could only do this based on a post from Paul Sokol 
I have just duplicated the Zap, and when testing it I’m hitting a 400 error. If there’s any chance anyone else has ventured into the same API/Zapier world, are you having any issues?
Error message below:
"Failed to create a request in Webhooks by Zapier
Invalid Refresh Token (HTTP Status Code: 400)"