Setting > My Home > Sense Monitor > Reset Data will wipe all your historic data.
Your options are:
Export the data prior to a reset as outlined here. i.e. Use the web browser login and export everything (at 1-hour intervals to maximize available data)
Read this (you should probably do 1. anyway)
If you really want all the old data you can (theoretically) scrape it … if you are truly brave and play nice along the way. I won’t post links here but there’s plenty of discussion on the forum about how to proceed there.
Add another Sense and keep the existing account. That would be my choice.