transition plan
A plan for moving from one system, state, or phase to another; often used in business, IT, and organizational contexts.
We formulate a transition plan to the new system.
We are proceeding with data migration according to the transition plan.
移行 is the general noun for 'transition' or 'migration'; 移行計画 specifically refers to the plan for that transition.
計画 is a general 'plan'; 移行計画 narrows it to a plan for transitioning.
Compound of 移行 (transition) + 計画 (plan).