Modern storage systems are often faced with complex trade-offs between the confidentiality, availability, and performance they offer their users. Secret sharing is a data encoding technique that provides information-theoretically provable guarantees on confidentiality unlike conventional encryption. Additionally, secret sharing provides quantifiable guarantees on the availability of the encoded data. We argue that these properties make secret sharing-based encoding of data particularly suitable for the design of increasingly popular and important distributed archival data stores. These guarantees, however, come at the cost of increased resource consumption during reads/writes. Consequently, it is desirable that such a storage system employ techniques that could dynamically transform data representation to operate the store within required confidentiality, availability, and performance regimes (or budgets) despite changes to the operating environment. Since state-of-the-art transformation techniques suffer from prohibitive data transfer overheads, we develop a middleware for dynamic data transformation. Using this, we propose the design and operation of a secure, available, and tunable distributed archival store called FlexArchive. Using a combination of analysis and empirical evaluation, we demonstrate the feasibility of our archival store. In particular, we demonstrate that FlexArchive can achieve dynamic data re-configurations in significantly lower times (factor of 50 or more) without any sacrifice in confidentiality and with a negligible loss in availability (less than 1%).