Somebody gloss this sentence please:
"As long as homeowners follow the existing maps last revised in 1984 they'll be grandfathered into the federal flood insurance program."
I've seen the dictionary definition for the verb grandfather, but I can't tell what the preposition into is supposed to mean in this sentence.
Dictionaries can be confusing.
Most of the definitions I found defined the verb grandfather in terms of ‘exempting’ someone from a regulation or ‘excluding’ someone from liability under a regulation. That's not inaccurate; but it doesn't really describe how the word is actually used.
As the answers at the ELU question Hellion linked point out, the term arose in the post-Reconstruction South, where in many jurisdictions those whose grandfathers had been voters—that is, all local white people—were exempted from the taxes and literacy tests required of others who wished to vote—that is, African Americans, who were recently freed slaves and the children and grandchildren of slaves.
The effect of these laws was that those who were exempt could vote and those who were not exempt could not vote. And that's how ‘grandfathering’ has worked ever since: you are exempted from a regulation which would put you in the unprivileged group, so you are, as a result, grandfathered into the privileged group.
In the case at hand, homeowners who would be ineligible for Federal flood insurance because their homes are in the wrong place on the new maps are grandfathered into eligibility if they had insurance when their homes were in the right place on the old maps.
Being "grandfathered into" in a legal sense means that a rule does not apply to something that happened before the rule was made.
An example: building codes. For example, the electrical code in the USA requires that all electrical wiring must meet certain standards. However, electrical wiring that was installed before the current code was enacted is "grandfathered into" the code and does not need to meet the new requirements.
In other words, existing electrical wiring is exempt from the requirements of the new code: a homeowner does not need to modify old wiring to bring it up to the standards of the new code, if the wiring was installed before the code took effect. The old wiring is grandfathered into the new code.
To be "grandfathered in" means you are allowed to keep doing something just because you've been doing it for a while already, even though you would not otherwise meet the new (stricter) requirements.
In your example they say "grandfathered into" because they are talking about moving you from outside the program to inside the new program (with all the permissions the program grants, some of which you were already using). If no program was mentioned later in the sentence, they would have said "grandfathered in".
