We need to keep the old file cache store around for the backward compatibility (JDG requirement).
Mircea, Randall's suggestion is compatible with what you're saying. The idea is that anyone that would use the FCS with Infinispan 6.0+, if FCS would be able to detect old FCS data structure, it could migrate it on the fly to the new single file based structure. That is backwards compatible and keeps the old file cache store code around, but transforms its inner details.
We'd also need a migration tool for migrating other cache stores (e.g. LevelDB cache store from certain users) so +1 for the migration tool.
Indeed, that might be needed, but for the specific case of the new FCS upgrade, you might not even need that, which is even better. If people want to migrate data from cache store X to cache store Y on a whim, then yes, you need a migration tool.
Disclaimer: I have not investigated yet the feasibility of Randall's approach.