Before:
Blacks could not exercise their right to vote, the were discriminated against in housing, employment, and were forced to live their lives as second-class citizens in much of the country.
Why conservatives opposed it:
It was a violation of "states rights", an interference in the marketplace, and most of all it was a violation of God's law.
Since:
Things have gotten much better for black people. Discrimination is illegal, and most importantly, those who are bigoted against blacks are considered social outcasts as well as being violators of the law. Conservatives today have come 180 degrees, and a few honest ones even admit they were wrong 30 years ago.

Before:
Most major American cities had severe problems with air quality. In Pittsburg and other places streetlights had to be turned on at noon the air was so bad.
Why conservatives opposed it:
An obious government interference in Free Enterprise!
Since:
The air has never been cleaner than it is today. Forcing facorties to use filters on their smokestacks has made a huge difference in the quality of life for millions of Americans.

Before:
Women could not vote. Period.
Why conservatives opposed it:
A woman's place was in the home. Politics was too rough for women. The Bible says so.
Since:
Women are actually considered equal to men these days. How radical.

Before:
Person A could not marry person B if person A was a different race. Why conservatives opposed it:
A violation of God's law. A radical re-definition of marriage. Immoral and unnatural.
Since:
Millions of couples who love each other have been able to get married. The Republic still stands.

Before:
70 hour work weeks. Child labor. Substandard wages. Company towns. Hired goons "talked sense" into employees who dared to think about joining a union.
Why conservatives opposed it:
Another example of government interfering in the free market.
Since:
The minimum wage, the right to join (or not join) a union, the 40-hour work work, child protection, a safer workplace, and of course a continuous (for 30 years after the law passed) rise in the national standard of living.

Before:
Caveat Emptor.
Why conservatives opposed it:
Another example of government interfering in the free market (sounds like a broken record).
Since:
Well, now you can rest easy knowing that those pjamas you bought for your baby won't catch fire easily and the swingset you bought your little boy has to be built so it won't collapse.

Before:
Rats.
Why conservatives opposed it:
Another example of government interfering in the free market yadda yadda yadda.
Since:
An outbreak of E-coli is a rare newsworthy event rather than a day to day risk.

Bonus! Today's candidates for the Conservative Hall of Shame
You can bet that in 25 years the items listed below will be added to the list above. Conservatives have a knack for being on the wrong side of history.