It is not possible to switch types.
What is the main issue(s)? How many projects have accumulated for each type? What are the areas of impact for switching besides history?
Usually you impact:
- History
- Phases
- Ongoing/time based
- Custom Fields
- having the type display everywhere (create project etc....)
Some customers that need to keep the history for projects of the old type will create a new field in the project type they wish to keep to designate the two project types and begin creating all projects under that type and slowly archive the now unused type over time. Unfortuantely there will always be traces of the type.