How to reduce user error while performing an irreversible action?

I have a control which manages the retention period of an email archive shown below.

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The customer is able to set a retention period of years and months. The max period is 10 years and the min period is 12 months. Reducing the period can possibly destroy emails kept in the archive. For example, reducing the Retention Period from 10 years to 4 years, will destroy all mail that have been in the archive for more than 4 years.

Feedback I have gotten on this control suggests the following:

  1. The behaviour of the control is not clear enough, which may cause mail to be mistakenly destroyed;
  2. The control is too easy to use, and thus it's too easy to make a mistake.

Can anyone offer improvements to the design of this control?

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