I'm pretty sure they are not backward-compatible, so it's unlikely there is a way to do what you want.
Typically .dproj is a netlist project and a .prj is a cdb project (especially with 2007.??) It appears that you will have to promote/degrade your 7.5 to cdb. No connectivity issues that I have seen, just must be carefully done.
You mention Version 9.2 so I assume you are talking about a PADS install/project. The version of DxDesigner in this install is the latest iCDB version of software (either a DxDesigner 2007.x or 7.9.x version). This version uses a binary database and a *.prj project file. Earlier versions of the PADS software used DxDesigner 2005.x or earlier which use ASCII schematics and a *.dproj or a '*.dproj and *.prj file' if using Expedition.
It is not possoble to open an iCDB based design file in the earlier DxDesigner 2005.x version of software (PADS 2007.3 and earlier).
If you are using the PADS flow also subscribe to the 'PADS Desktop PCB Design' community http://communities.mentor.com/community/pcb/pads?view=discussions