Go abroad, or move
Can I change units or currency later?
Yes, any time, in Settings → Region (Units, Currency, Date & time). The log is stored in miles, US gallons and the money as paid, so a change only changes how it’s shown: nothing is converted for good or lost.
Your units are yours; the display follows What you type and what’s stored never change with the display.
Exports, money and tax
Each log’s CSV export follows the new units; Export everything, the backup, keeps miles and US gallons, so it restores the same anywhere.
A new currency shows every amount at its own day’s rate (looked up when you’re online). A new region asks before it changes your currency, and says when the new one turns the tax tools off (the IRS’s rates and the tax package go by US dollars, CRA’s by Canadian dollars).
A business mileage rate keeps the currency you typed it in.