The Web Application Proxy (aka WAP, that's how we call the ADFS Proxy since Windows Server 2012 R2) does not leverage the ADFS service account at all.
As a matter of fact, WAP don't even need to be domain joined. WAPs authenticate with the ADFS farm using TLS authentication (certificates are generated when you join the WAP to the farm and then roll-over on a regular basis).
In other words, there is no action required on the WAPs when you change the service account of the ADFS farm.