Index upgrade from 1.4.1 to 3.1 and 4.0

[Ofer Fort]

Hi all,
While doing some tests, I realized that an index that was created with
solr 1.4.1 is readable by solr 3.1, but nt readable by solr 4.0.
If I plan to migrate my index to 4.0, and I prefer not to reindex it
all, what would be my best course of action?
Will it be possible to continue to write to the index with 3.1? Will
that make it readable from 4.0 or only the newly created segments?
If I optimize it using 3.1, will that make it readable also from 4.0?
Thanks
Ofer