As far as we know, it should be possible to handle both .wpost and wpostx files in Microsoft Windows Live Writer desktop publishing application, so it may be very well possible to resave the older format to the newer version. You can download the program from Microsoft Website.
Convert Microsoft Windows Live Writer post file to Microsoft Windows Live Writer post.
We found one software available in the database for this conversion.
Microsoft Windows software - convert wpost to wpostx on Windows
Progam for making articles on popular blog services
Convert from wpost file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.