No, It is not possible to downgrade the EOTE project to an older version.
But when we upgrade the original project, for example, when we upgrade the project from EOTE 3.1 SP1A to EOTE 3.2, EOTE will create a back-up project file. we still can open the back-up project with EOTE 3.1 SP1A. So please keep the back-up project properly if needed.