Julie Koesmarno has a Q&A on Azure SQL Database:
Q: Is there going to be down time when I scale up/down? What’s going to happen to my existing connections?
Extracted from Change the service tier and performance level (pricing tier) of a SQL database:
Note that changing the service tier and/or performance level of a database creates a replica of the original database at the new performance level, and then switches connections over to the replica.No data is lost during this process but during the brief moment when we switch over to the replica, connections to the database are disabled, so some transactions in flight may be rolled back. This window varies, but is on average under 4 seconds, and in more than 99% of cases is less than 30 seconds. Very infrequently, especially if there are large numbers of transactions in flight at the moment connections are disabled, this window may be longer.
The duration of the entire scale-up process depends on both the size and service tier of the database before and after the change. For example, a 250 GB database that is changing to, from, or within a Standard service tier, should complete within 6 hours. For a database of the same size that is changing performance levels within the Premium service tier, it should complete within 3 hours.
Video by Joe Idziorek on Service Tiers and how to scale up and down using Azure Portal is available here.
Read the whole thing. There are some great questions and answers in this set.