There is a community contribution that helps with this by converting one datatype to another, but in general it is not that easy. http://ez.no/community/contribs/datatypes/datatypes
It may be a better alternative to adjust the classes so that they have the same attribute names, and then use the same override template for both classes. Then, if you take one of the two classes out of the content group, it cannot be used to add any more content in the future, but it will not affect the existing content.