In our previous post, we walked through the agent mesh for software modernization architecture we built on Red Hat AI for modernizing legacy systems at the scale that mission environments actually require. The post covered the harness pattern, the workflows the harness orchestrates, and the KPI framework for measuring success. It also explained why we run coding agents on Devstral and non-coding agents on Ministral, inside a platform that operates in disconnected environments.This post is about the next layer of that story: the pluggability.In the regulated environments where agent mesh for software modernization belongs, the question of which model to run is never a one-time decision. Codebases run to millions of lines while defense and financial services environments are often disconnected. This means whatever runs the workload has to keep running without an outside API call. The model landscape keeps moving, and at some point one of those moves will involve swapping out the model powering your coding agent entirely. The architecture has to absorb that without rebuilding the iterations around it. That is what makes pluggability structural rather than aspirational.In agent mesh for software modernization, the model is a component, the agent is a component, and the static tools around them are components. The model swaps in through the serving layer. Agents and the static tools they call sit behind clear interfaces, so a connected commercial coding agent (e.g. Claude Code or Codex) can run early when policy allows it, or it can be swapped for an on-premises alternative like OpenCode or a custom agent without rewriting the orchestration above. The iterations of the harness, the automated workflow tracking and merge policy, and the audit trail stay stable across those swaps. Teams can start on the components that fit today and absorb what comes next as upgrades rather than rebuilds.This post walks through what that looks like in practice when we upgraded to Gemma 4 and the latest CrewAI in parallel. We cover where the swap was clean, where it required scoped engineering work, and why this property of the architecture matters more in regulated environments than it does anywhere else.Pluggable by design