Signing in won't remove anything: your snippets stay on your phone.
Your data stays on your device
All of your 1SE content is stored locally on your device. That means signing in won’t overwrite or remove anything. Your snippets, journal entries and projects stay exactly as they are.
Signing in doesn’t back up your snippets. If you delete the app, lose your phone or switch to a new one without a backup, your content could be lost.
To back up, see How to Back Up with iCloud (iOS), or use 1SE Cloud Backup with a Pro subscription: see How do I back up all my projects?