There are two solutions for same.
First Method: Keep question format as multiple select only and apply mutual exclusive in all options. This will ensure that only one answer can be selected - single select format.
Advantage of this method is that your data format will remain same and no data is lost.
Second Method: Make a new question in single select format and apply a "always skip" in existing multiple select question. While data preparation, write code in spss which can pick data from old variable.