Here is Authorize.net's announcement of the update for their sandbox (test) servers: http://community.developer.authorize.net/t5/The-Authorize-Net-Developer-Blog/Authorize-Net-Begins-Infrastructure-and-SHA-2-Certificate/ba-p/49615
Here is Authorize.net's announcement of the update for their production (live) servers: http://community.developer.authorize.net/t5/The-Authorize-Net-Developer-Blog/Production-Certificate-Upgrades-begin-May-26-2015/ba-p/50430
Note that according to the above post they are also updating/changing the IP address pools for their domains names. ShopSite uses a domain name to connect to Authorize.net, not a hardcoded IP address, so this change should not affect ShopSite merchants either, unless your host has firewall rules that limit outgoing connections to certain IP addresses. If you experience issues connecting to Authorize.net, ask your host to check their firewall rules to make sure connections to all secure.authorize.net IP addresses are allowed.
Last Updated
30th of April, 2015