You have a current iModel which was created using an OPM Workspace and check-out and in objects using OPM. The OMP workspace changes where an attribute type changes from a DOUBLE to a STRING, there is no way to update the iModel, the only option is to start a new iModel from scratch.

To have the ability to update the database attributes in an iModel to be able to change attribute type or any other modification from OPM workspace where it involves update of the attributes, removing an attribute being another.