How can I use different Forms for old and new Items of same ItemType?

Hi, I have a custom Change ItemType that is already used for around 50 change processes. Now I plan to extend this ItemType with a few additional features. This also requires many changes in the ItemType Form. The current Form is used as Default Form. Any change I apply to the Form, will also be visible in the existing 50 Items. But I would prefer, if the previous Items still use the previous Form. What´s the best way for using the old Form for old Items and the new Form for new Items? My current ideas: 1. My custom Change ItemType doesn´t use the classification property. It´s maybe possible to use the classification property to store information about the verison. Then it would be possible to separate the old and new Items according to the classification. 2. Using some onShowItem Client Method. What would you recommend? Do you know any other way to achieve this task? Thanks for your help! Angela