Although I tried to cancel the update, all the rain data were already lost. So I continued the update, and "recreated" the rain data from the data collected in Weewx. The barometric calibration was also gone. Fortunately, I have my data saved in a spreadsheet, which calculates them for me (see attached worksheet). Timezone was also reset to UTC 0. And except of updating Ecowitt.net all connections were lost.
After activating interceptor I immediately got a message in the log file Warning ... LOOP value 'rain' 17.196 outside limits (0.0, 10.0)
Fortunately, there was no rain data to collect, so I ignored it. But, may be the rain data need some supervision in Weewx after this firmware update. Don't know, if this was an interceptor specific problem. Maybe I should consider now, to switch to the new GW1000 driver.