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Noticed this the other day while reading a prospectus for the Vanguard NY Long-Term Tax Exempt Fund. Interesting (Admiral class shares have a slightly lower expense ratio), but I won’t have to think about this for quite some time…

[Tenure Conversions into Admiral Shares]

Three-year privilege. After three years in the Fund, you may convert Investor Shares into Admiral Shares if your account balance in the Fund is at least $150,000 and you are registered with Vanguard.com.
Ten-year privilege. After ten years in the Fund, you may convert Investor Shares into Admiral Shares if your account balance in the Fund is at least $50,000 and you are registered with Vanguard.com.