Is it possible to change the secondary database from warm standby/read only to normal mode? Will it stop the logshipping?
Yes, it is possible to change the secondary database from warm standby/read only to normal mode.
You can run a command on the secondary database to do this to take it out of standby. (restore database my_db with recovery)
It wont 'stop' the logs from shipping. You need to pause/stop the SQL agent jobs to do that.
If you mean will it 'break' the log shipping process, yes it will if you switch the secondary database out of standby to normal mode.
You will then have to reinitialize the log shipping definition from the primary database.