Will there be any down time when we change our site DNS entry to the WAF IP address?

Yes.  This change can cause up to 1 hour of down time for your website while DNS propagation, HTTP validation and SSL validation take place.   So please take that into consideration when planning/scheduling to update your DNS A Record. 

Until the validation process completes, your site may display a DNS resolution error (Error 1001) or "Connection is not secure" error (ERR_SSL_VERSION_OR_CIPHER_MISMATCH), as shown below.  This is expected, so please be patient and allow up to an hour before contacting ASI Hosting Support regarding these errors.