As Oracle has stated this will be the last release of HFM that will require a full software installation (to their knowledge, safe harbor statement applies). Other updates after this will be via patch set updates. To do this installation, you cannot (or should not) just update the software on the existing servers. The software will need installing on a clean server, the database copied over and upgraded, interfaces tested, and then consolidation and data verified. A parallel close would be a prudent step.

To plan for this, think about budgeting for the new servers now. Most companies are in the middle of putting together their annual budget for next year, so now is the perfect time to plan this upgrade. Unless some design changes are incorporated at the same time, no consulting help should be needed unless it is to help with the infrastructure setup. If you are on an older version or you're having to do work-arounds to make things work with your current design/setup, then you should think about taking this opportunity to do an "application remodel."

The other option would be to plan a migration to FCCS, Oracle's cloud based consolidation and close solution. In this case you would be budgeting for an operating expense, not a capital expense, and not having to budget for the annual maintenance on the on-premise software. Migrating to FCCS will go much easier with some consulting help, but with the recently released HFM to FCCS migration tool, it goes more smoothly than before.