Usually, you can open the .bkp file with the newer version of Aspen Plus. It then asks you if you want to keep all features of the newer version or keep upward compatibility intact. If you plan on using the file in the older version again afterwards, choose option nr 2.
Sherida is correct. You can open the .bkp file in the newer version. The .APW (workbook files) dont work in newer versions which is a bit poor from aspen really. So much for upwards compability !.
If you have .apw files dont worry too much - normally aspen saves a .bkp as well as a .apw file.