The trend and best practice would be to leave the projects separated, yet with the SAME structure (custom fields, lists etc.).
HP has added various features over the years to the Premier/ALM level product that help in these complex multi-project environments:
- Project Templates
- Entities sharing across projects
- Cross-Project reporting
Another driving-force behind NOT using ''one big project'' is that with one project, you are exposed to a large disaster if something should get corrupted rendering the project un-usable. When you break the work into multiple projects, this spreads out the risk so that if any, only one project fails and can be remedied fairly quickly from a backup.
Although it is possible to combine projects (with lots of copy/paste operations), but some links between entities and other subtleties may be lost or not transferred.
HP offers NOTHING to help you do this other than copy/paste of selected entities from one Internet Explorer-instance/project into another, or various dump to external data files, massage, and load back in with Excel AddIn.
If you really want to do this, perhaps you start a NEW effort with the ''one big project'' approach rather than shake up existing production when you try to combine them - ii will be a MAJOR construction zone as you do this combine operation.
In HP's best practices doc ''Project Topology best practices'' PDF, in response to the ''one big project'' approach, HP says:
''HP would not advise implementing this scenario.'', then gives[link text] MANY reasons why not.