It should be possible to see goal conversions that happend before the goal was created in piwik if there are fitting ulr patterns in the historical data.
Keywords: goal historical
Re-processing the logs might be useful for some, but is potentially very intensive (memory, cpu, i/o) to run. It also requires archive tables to be regenerated for the same period.
Another example is reprocessing referers (for search engine and keywords), when a new search engine is added.
as vipsoft explained, this is too complicated / risky.
Shame. I would have liked this.