I believe that one of the issues is the depth of the existing window frames - if they are too shallow, they may not be able to accomodate double glazed units. Or at least, that is the gist of what I understood when we considered this several years ago. We tried a few places but they were reluctant to consider it (we evetually got in new windows).
Having said that, it is certainly worth checking out as it is likely to be cheaper, if it proves possible, as well as keeping your existing windows out of landfill. Good luck.