Hello,
There are a few different options here. I'd recommend first to consider using classification-based forms. This way you can get a proper form appearing using OOTB functionality when the user changes the classification of the ItemType.
One other thing you could do is open the part form in a new tab with aras.uiShowItem.
If you need to completely replace the form, I've found that the following code works from an onClick event of a button at least in R33.
[embed:dc8ab71f-3b98-42d9-b0f6-e21e02a0f8e2:3f9cbae6-2a8a-4d4f-a625-f151b016c8b5:type=javascript&text=const%20partNd%20%3D%20aras.getItemById%28%22Part%22%2C%20%220BB5120838414E36ABAA90DB866B293D%22%29%0D%0Aaras.uiShowItemInFrameEx%28window.frameElement%2C%20partNd%2C%20%27view%27%29%0D%0A%2F%2F%20window.frameElement%20is%20what%27s%20needed%20to%20replace%20the%20existing%20form%20from%20the%20context%20of%20the%20onClick%20event]
Be aware that the form will be re-drawn to the original if you edit/refresh/etc.