Be sure you make backups of Kamailio and Siremis databases. Also, you can backup the directory where Siremis was installed. In this way, you can restore back if something goes wrong.
Some notes and tips about upgrade of an existing v3.2.1 installation:
Siremis 3.3.0 continues on top of 3.2.1, the upgrade should be straightforward
copy the files (not the folders) from siremis-3.2.1/* to siremis-3.3.0/*
copy siremis-3.3.0/siremis/modules/ser over siremis-3.2.1/siremis/modules/ser
with a web browser, go to Siremis administration, under Modules menu, reload ser module