The issue with changing the Item type and why it is hard is because of attributes. Attributes required on one type may not be required or even exist on the other type. Mapping the attributes while converting types is the biggest issue. Most companies that have a number of Item types usually have to write ITK to convert from type to type. As we all know, Users regularly create the wrong type and think an admin can simply change it for them. Not the case without a custom program built specifically for that purpose.
There may be some migration tools that you can leverage for your task, like csv2tcxml (a.k.a. bulkloader), but I encourage you to write ITK instead as this will be an ongoing effort and not just a one-time swap. The more Item types you have then the more this utility will come in handy (ROI).