do you get downtime when you increase Azure SQL DTUs from 10 to 20?

you don't need downtime when you Increase DTU's or change service tiers,I have changed many times on my test instances as well.

Our database is nearly 150 GB and we have changed service tier from P1 to P3,this operation completed in about 15 minutes and there is no downtime.I remember there was a MSDN page which gave a formula on estimated time ,but couldn't find it now..and out tier change completed much earlier than estimated

Some transactions may get rolled back during the process

https://docs.microsoft.com/en-us/azure/sql-database/sql-database-dtu-resource-limits-single-databases

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. The length of time for the switch-over varies, but is less than 30 seconds 99% of the time. If there are large numbers of transactions in flight at the moment connections are disabled, the length of time for the switch-over may be longer.