This article is from the FAQ, by with numerous contributions by others.
No, you will not lose your permanent resident status if you have not yet
applied for a replacement Green Card, nor will you be penalized if you have
not yet applied for the new card. However, lawful permanent residents are
required by law to carry evidence of their status, and expired I-151s no
longer meet this requirement.
A person can still apply after March 20, 1996, and should still apply as
soon as possible for a replacement card to avoid difficulties in obtaining
employment, entitlement benefits and reentry into the United States from