How do you "upgrade" a data center running an e-commerce web application that serves 100 million customers? Verrrrrrry carefully. If you're Paypal, you make sure to inform your developers and merchants about the maintenance window. The PayPal team is alerting partners that it is "planning to upgrade the PayPal data center" on June 18th and will take the site offline for two hours:
During this time, there will be a PayPal system-wide service interruption. The PayPal site and all APIs will be unavailable. You will not be able to process payments during this time. You do not need to do anything during the data center upgrade.
That is, unless you've hard-coded PayPal's IP addresses, which are going to change during the transition (see the list of PayPal IPs for updates). The company didn't provide details on what was being upgraded. PayPal is part of eBay, which averages 1 billion page views per day on its web infrastructure.