Possible reasons:  

New (changed) business requirements

Low performance

Unreliable functioning

High cost of support

Low quality /absence of  documentation

Update/Fixing Oracle Database Application Service

     Check & Analysis:

Logical database design

Physical database design

SQL performance

Design & performance of PL/SQL procedures

Database instance (s) performance

     Possible Solutions:

Database logical & physical design refining

Re-engineering & code study for documenting

Data cleanse & fixing

Re-design/creation database procedures

Instance(s) tuning

  Examination for Compatibility with:

Naming policy

Security policy

Existing data flows

Existing error processing

     Solution Build:

Setup development environment

Update database design (DM metadata)

Generating DDL & deployment

SQL & PL/SQL coding

Preliminary testing

     Updates & Fixes Testing:

Setup testing environment

Moves updates development environment

Setup agreed test plan

Test plan execution

     Delivery & Maintenance:

Deployment in production environment

Supervision of updates running

Full agreed set of documentation

Three months free maintenance of updates

Further maintenance for agreed price