The C3P file is a zip file containing text files.
Decompress the content of your file, open "project.c3proj" in a notepad.
In that file, you have first the line "savedWithRelease" that you need to change the value of to the release you want to open your project in.
ex:
"savedWithRelease": 10400
"savedWithRelease": 98000
to change from release 104 to 98.
Then you will have to modify the "version" value for all depicted "usedAddons" as well.
ex:
{
"type": "plugin",
"id": "Sprite",
"name": "Sprite",
"author": "Scirra",
"version": "r104",
"bundled": false
},
{
"type": "plugin",
"id": "Sprite",
"name": "Sprite",
"author": "Scirra",
"version": "r98",
"bundled": false
},
to change from 104 to 98.
Once done, save this file, recompress into the c3p file and then you can try and open it up in a previous release.
It will work only if you are not using specific features that were implemented after the older release.