Amending previously filed returns won't generally change the statute of limitations for assessment (not sure about collection without some research). It might or might not attract more scrutiny from the IRS for the amended or other years. There is an exception for amended returns filed within 60 days of the limitations date. For those the IRS gets an 60 days from the date the amended return is received even if that is beyond the original limitations period. See I.R.M. 25.6.1.9.4.2(1) https://www.irs.gov/irm/part25/irm_25-006-001r-cont01.html ; the actual rules for the limitations are fairly complicated and have a number of exceptions that don't usually apply. See https://www.law.cornell.edu/uscode/text/26/6511 for the details.
The IRS can actually go back forever to adjust returns, it's just that they can't assess new tax usually after three years (six years for substantial understatement of income, forever for fraud or non-filing). Why would they go back further if they can't get more tax? Because carryforwards might affect open years.
Also there is no duty to amend a return. Google "duty to amend tax return" for many articles on it. You may want to amend to get money back or to reduce interest/penalties if the service catches up to you, or just to get it right, but it appears that if you made an unintentional mistake you are not required to amend.
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