The IRS says that you should fix mistakes, but you are not legally required to do so. Instead, you should weigh the risks of not fixing the mistake against the benefits of amending the return. Amending a past tax return is very common.
Often, if you failed to report all your foreign income or assets, then the risks are high, and it makes sense to amend. Depending on your situation, you may get the best result from amending under an IRS amnesty program, such as a Streamlined Filing. If you used a mass-market software or a tax preparer who wasn’t familiar with international issues, then your situation is high risk!