Even though it would only involve a conversion of an existing film, Disney has decided to shelve their project to convert their 1991 release of BEAUTY AND THE BEAST into 3D. But if Disney has the technology available, then why not go for it. However, it appears that the process is very involved and costly to convert existing films to a 3D style format. But, you'd expect software to be developed that can make 3D film conversion more widely available at some point in the future. However, now it appears that a painstaking frame by frame new animation art process is required.
Will new 3D versions of existing films ever see the light of day? It might depend on the process used and the type of technology. But you can expect Disney to be at front line of new technology development, and newly enhanced versions of existing films will certainly be released in future years giving new generations the opportunity to enjoy these films.