'project_time_mode_id' is the field used to determined the UoM to use when logging timesheets. It is not used in project alone, but in timesheet. Moving it make sense, and does not cause any problems. For now, no UI allow the end user to change it: it is set with the Hours UoM (hardcoded). This is a technical move to prepare the feature that will allow people timesheeting in Days or Hours. Task #39079