If you make a factory reset, you get back to the version with what you have flashed the phone long ago. All later updates will be lost. But as long as SFOS 4.5 isn’t yet rolled out officially but only EA, you can download the 4.4.0.72 flash image from the Jolla shop homepage and reflash with it.
If you do so and later make a ‘factory reset’ you come back to 4.4.0.72 then and not to older version with what the phone was flashed initially.