Worked for me, I used auditing on Investment Rollup, and it had to be on Insert and Delete, and I audited both Child ID and Parent ID. The Insert is to track when a Project is assigned to parent. The Delete is to track with a Project is removed from a parent.
There are challenges with it, for example, creation of a Project will automatically create an audit record, even though the Project hasn't been assigned to a Parent. Ignore these records.
The only way I could see this data is via Administration/Audit Trail, and then in my case, filtering by Object: Investment Rollup.