LeadSight only ever requests read access to your Xero invoices and contacts — enough to match leads to closed, paid revenue.
LeadSight cannot create, edit or delete anything in your Xero account. It's a one-way, read-only connection.
LeadSight imports Xero invoices dated on or after the day the tracking script was first detected on your site, so revenue is tracked from the day the script goes live. Install the script before connecting Xero; the first sync starts once the script has been detected.