The Battle Lab Collaborative Simulation Environment (BLCSE) federation is the Army Training and Doctrine Command's (TRADOC) biggest federation to serve the Army's analytical community. BLCSE has a large, complex, federation-of-federations architecture consisting of 29 different constructive and virtual simulations at 14 geographically distributed sites. The current BLCSE technology environment is comprised primarily of the Distributed Interactive Simulation (DIS) as the primary inter-federate communications protocol. DIS interoperability standards were developed in the late 1980s to support the linkage of simulations exchanging low entity-count data, principally entity-state messages between virtual training devices (e.g., SimNet devices). Active entity counts within BLCSE federations have been steadily increasing as federations grow to support more comprehensive analyses. BLCSE has reached a point where DIS protocol communications cannot reliably manage the federation message load without an externally managed message distribution management scheme. The effects of DIS message saturation, either on the network or at the application itself, are lost messages or incorrectly sequenced messages. Both problems lead to entity state anomalies and lowered data reliability. In view of these challenges, Army Capabilities Integration Center's (ARCIC) Simulations Division Director approved a Simulations Division initiative, in May 2005, to transition the BLCSE federation from DIS (IEEE 1278) to Higher Level Architecture (HLA -IEEE 1516) interoperability standards. However, TRADOC plays an important role in the Army's Cross Command Collaboration Effort (3CE) organization. The 3CE organization currently adopted the Department of Defense (DoD) HLA NG 1.3 standard. In order to provide interoperability with 3CE federation, BLCSE had to implement the NG 1.3 protocol as an intermediate solution. After a year and a half of effort, 20 BLCSE federates are able to communicate in the HLA 1.3 environment. To complete the projects' goal, the development group worked on BLCSE's transition from HLA 1.3 to IEEE 1516. This paper describes the challenges, issues, and problems uncovered during BLCSE's ongoing transition from HLA 1.3 to IEEE 1516 and the lessons learned for transitioning future large-scale, rapid-growth federations.