The upgrade from Alfresco's SOLR1.4 to SOLR4 search subsystem requires a complete re-indexing of the repository content to build the new SOLR4-compatible indexes. To maintain search availability during this potentially long process, Alfresco is designed to keep the existing SOLR1.4 subsystem active for handling queries. Consequently, only the functionalities supported by SOLR1.4 are available to users. Advanced features introduced with SOLR4, such as improved filtered searches and facets, will only become functional after the re-indexing is complete and the system has been switched over to use the new SOLR4 index.

A. While facet configuration is required, its absence would affect specific filters, not disable the entire filtered search capability, which is the symptom described.

B. Index corruption is a possible failure but not the expected, designed behavior of the upgrade process. It would likely result in broader search errors, not just missing features.

D. This statement is factually incorrect. SOLR4 offers significantly enhanced filtering and faceting capabilities compared to its predecessor, SOLR1.4.

1. Alfresco One 5.0 Documentation, "Upgrading from Solr 1.4": This official documentation outlines the migration process. It states, "When you install Solr 4, the Solr 4 search subsystem is not enabled by default. The existing Solr 1.4 subsystem continues to serve search queries until the Solr 4 indexes are built." This confirms that the system falls back to the older search service during the re-indexing phase. (Source: Alfresco One 5.0 Documentation, Administering -> Search -> Solr 4 search subsystem -> Upgrading from Solr 1.4).

2. Alfresco Content Services 6.2 Documentation, "Upgrading Search": While for a newer version, the principles of major search version upgrades are consistent. The documentation for upgrading search subsystems emphasizes that a full re-index is necessary when the underlying index format changes significantly, as it does between SOLR1.4 and SOLR4. During this re-index, the new subsystem is not yet active. (Source: Alfresco Content Services 6.2 Documentation, Upgrading -> Upgrading search).