Is it possible to change an existing SQL Server installation to SQLite preserving all existing data? I have a low-traffic forum currently installed with SQL Server but having it in SQLite will make my live easier for maintenance... unless you think is not a good idea.
Thank you!

Its definitivly possbile, and with low traffic it should be no problem.  

I found multiple converters to Convert an MS SQL database to an Sqlite db. So it should be easy to convert all data.

Added a quick guide on how to migrate..
https://github.com/YAFNET/YAFNET/wiki/Migrate-DB-from-MsSql-to-Sqlite