On my farm solution, I am creating custom Site Collection Definition, Content Type and new List based on that Content Type.
For the List, I created custom form and added it to the list Schema.xml as follows
<Form Type="NewForm" Url="NewForm.aspx" SetupPath="features\$SharePoint.Feature.DeploymentPath$\PostList\NewPost.aspx" WebPartZoneID="Main" UseLegacyForm="TRUE" />
Where "PostList" is my custom list, and "NewPost.aspx" is my custom add new item form.
Now this custom form is new thing I added, the previous version of my WSP is already deployed and it created the Site Definition, Content Type, List Instance. When I do upgrade using (stsadm -o upgreadesolution...), the form get deployed in the 14 hive but when I click add new item, the popup opens with disabled ribbon and nothing in its content.
I tried deleting the Site Collection and creating it again using my custom Site Definition, it works fine and clicking add new item gives me my custom form. However, I can not delete the site collection and recreate it in the production, can anyone let me know what is my problem with this custom form and upgrade?