To answer your question, no, you can switch audio sources in the Sonos app without doing any sort of reset.

That said, you should know that you will need the line in adapter that Sonos sells separately to use the line.  Don’t use 3rd party adapters as they are converting digital to analog, not analog to digital, as the Era 300 needs.  Secondly, the phono output won’t send a good signal to the Era 300.  If your record player has a line out, use that connection instead.  If not, then you will need a preamp between your record player and the Era 300 to boost the audio signal.