A challenge that is commonly overlooked when upgrading EPM is keeping your upgraded application in sync with your Legacy application after the initial upgrade, prior to go-live. The challenge comes from the requirement to continue day-to-day operations in the Legacy environment during the upgrade project time line. At some point, the Legacy application must be "upgraded." The upgraded application then becomes a snapshot, or a "Point in Time" of the Legacy application; any changes made in the Legacy environment beyond that point must be accounted for in some form or fashion in the new, or upgraded environment and application.
Project time lines differ, but there are standard project milestones that virtually all EPM Upgrade Projects must pass before going live in the upgraded environment; Application verification testing and data tie out, UAT, SIT, and at least one parallel for HFM applications. These milestones typically take 1-3 months, during which time the Legacy applications are being used for day-to-day operations; data is being loaded and consolidated or aggregated; metadata is being updated; forms, calculations, and rules files may be updated; reports may be create and altered; etc.
For an HFM upgrade, it may be as simple as performing a final "upgrade" of the Legacy application prior to go-live. This approach would require additional, high level validations testing. It is also dependent on the Source and Target versions, as some upgrades require an interim environment. Some EPM components and versions have a more elaborate upgrade process that does not lend itself to being repeated.
Tracking changes in the Legacy environment and repeating those in the new environment is another option. And some application components can easily be exported and imported between versions; Calculation Manager Rules, Financial Reports, Planning Web Forms, Essbase data.
In the end the EPM components in play, along with the delta between versions and the changes that need to be accounted for will contribute to the unique challenges for a given upgrade. The take-away is to recognize the challenge exists and to allocate time and resources to meet the challenge.