Sql-server – Install SQL Server Express without administration rights

Unfortunately, you cannot install SQL Express without Administrator rights on the machine and it's not something you can set in the config file or on the command line. If being able to install without Administrator rights is a requirement, you may want to investigate using SQL Server Compact Edition.

See the following MSDN topics for more information:

• Using SQL Server Compact 3.5 (Visual Studio)
• How to: Deploy a SQL Server Compact 3.5 Database with an Application
• SQL Server Compact 3.5 SP2 Books Online
• Local Data Overview
• Choosing Between SQL Server 2005 Compact Edition and SQL Server 2005 Express Edition -- A little dated now, but much of it still applies.

As Shark already pointed out install or downgrade are distinct actions:

• install side by side means to have a new session of Express edition installed WHILE the current running edition is working (btw, what is the version of the current instance?);
• DOWNGRADE/UPGRADE means running the installation option over the currently running instance;

Any operation involving the live instance means it will interact and stop with the current instance. Some planning questions are in order now:

• What are the dimensions of the database(s) that are in work? (Express edition can handle small dbs only);
• What hardware necessities do you have for your current load? (Express edition has some strict limitations in CPU, RAM, hdd space);
• What features do you need from the current set? (eg: you won't have SQL Agent jobs with Express Edition);

At first you need to answer all these questions and only then see what your options is.

EDIT: after all your details specified, I'd suggest you to install a new version along with the current working one. Then stop the working instance service, ATTACH the database files to the Express Edition, then move on with the uninstall of the other instance. This will make the database unavailable only for the attach operation.