When a high court rules on a matter, when does the precedent take effect? Say a case is in progress in a lower court which asks the same question as what was just decided on the same day in the high court. I know most cases span months sometimes. Do whole cases start right over from scratch when the high court sets a new precedent? Do cases pause to wait for an answer? What if two cases simultaneously ask the same question?
tl;dr: Precedent takes effect on the decision date.
You didn't list a jurisdiction, so I'll give a U.S. example. In Citizens United v. FEC (U.S. 2010), there are three dates listed in the header:
- Argued March 24, 2009
- Reargued September 9, 2009
- Decided January 21, 2010
Precedent attaches to a decision date. That's also why we see 2010 in the case citation above. However, it's important to note that not all precedent is binding. A decision made in the 7th Circuit isn't binding on a district court in the 2d Circuit, and vice versa.
As to the impact on cases in progress, the party that is more favorably impacted by the change can bring it to the judge's attention. The outcome will depend on the a) whether the decision is relevant, b) whether the decision is binding, and c) how far along the case at bar happens to be. Note: most U.S. cases never go to trial. It is (relatively) lower stakes to incorporate new legal theories in the pleading or discovery stage.