Veja o que significa a expressão, segundo a Wikipedia (http://en.wikipedia.org/wiki/Grandfather_clause):

Grandfather clause
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In American English, a Grandfather clause is an exception that allows an old rule to continue to apply to some existing situations, when a new rule will apply to all future situations. It is often used as a verb: to "grandfather" means to grant such an exemption. For example, a "grandfathered power plant" may be exempt from tougher pollution laws.

Often, such a provision is used as a compromise, to effect new rules without upsetting a well-established logistical or political situation. As well as being "grandfathered in" to avoid suffering new penalties, people may be "grandfathered" to receive new benefits they are not otherwise entitled to. For example, if a company has a pension plan that after a certain date improves to include additional or superior benefits, those who have already retired may be "grandfathered in"; they may be granted the same improvements to their existing pension plan, so that they are not, in effect, penalized for having retired prior to the rule change. This amounts to the same as the new rules being "not retroactively applied".

Acho que o que mais se assemelhe é a instituição do "direito adquirido", garantia constitucional segundo a qual a lei não pode retroagir para alcançar situações anteriores à sua vigência.