Do I file both the FBAR and Form 8938, or does one replace the other?

Neither replaces the other; they are separate filings with different rulebooks. The FBAR (FinCEN Form 114) goes to the Treasury's Financial Crimes Enforcement Network and reports foreign financial accounts when your combined balances top $10,000 at any point in the year. Form 8938 goes to the IRS with your tax return under FATCA and covers a broader set of specified foreign assets, with higher thresholds that vary by filing status and whether you live abroad. Many people cross both thresholds and must file both, often listing the same bank account twice. The definitions of what counts differ, so do not assume filing one satisfies the other. The IRS publishes a side-by-side comparison chart on irs.gov. Both are information returns and neither, by itself, creates a tax bill.