Is it ok to change from full recovery to simple recovery in Sql Server
I would do this when the database is in low activity (end of day, overnight, first thing in the morning). I would NEVER recommend making changes like this to a live database in the middle of working hours. It's probably a good idea to change the database to read-only or make sure no users/applications are hitting it.

Run a full backup of the database/log file in question, while still in full recovery mode. This will give you a starting point in case anything gets borked and you can just restore it.

Switch to SIMPLE recovery, I don't like using the GUI, I'd script it out.


Here's a great article from MSDN about things to consider when changing your backup model:
Considerations for Switching from the Full or Bulk-Logged Recovery Model.