Sorry to bother you and thanks a lot for your help and opening the project with the version last saved is the only way to save my previous projects as you suggested.
When I look at project.c3proj file, I found the following lines.
"projectFormatVersion": 1,
"savedWithRelease": 30802,
It seems to be last saved with 308.2
When I tried editor.construct.net/r308.2, it says not found
When I tried editor.construct.net/r30802, it says not found again.
When I tried When I tried editor.construct.net/r308, the old editor loads but can't open the project since it was saved with new version of "308.2".
How can I load the old editor with 308.2?