Provided are a computer program product, system, and method for applying a platform code level update to update a source partition in a computing system. Computational resources in the computing system are allocated to a destination partition. A code load is applied to the destination partition to implement an updated platform code level comprising an update to the platform code level on the source partition while the source partition is operational and processing computing requests. Submission of new transactions to the source partition is blocked in response to applying the code load to the destination partition. An operational environment and system state at the source partition are migrated to the destination partition in response to blocking submission of new transactions to the source partition. Transactions are directed to the destination partition intended for the source partition in response to migrating the operational environment and system state to the destination partition.