I've built a custom content type called "Document for Review" and added a few extra fields (as you'd guess it inherits from Document). However I'd only want some of those extra fields to show up when any item from the content type is edited (some are to be fixed at insert). Searching out there hasn't given me a definitive answer to this. This also has to be a sandboxed solution (if not done straight out of SP Designer) as it will be running on SharePoint online.
Is you SharePoint Online account type dedicated or multi-tenant. If multi-tenant then it will have to run in Sandbox but if dedicated you can get your solution approved from MSO and take advantage of full SP object model?
You can try to customize the list forms as InfoPath forms using SharePoint 2010 designer and in InfoPath you have the liberty to create rules, so you might be able to create rule to check if "Edit Item" form is being opened then only show certain fields. Refer this URL to customize list forms:
Check if in Sandbox execution model InfoPath forms are available or not?
You need to do the following:
- Update the settings of your Document Library. Make sure to make the fields you want to be viewable when editing to be optional fields. This can be done by going to List Settings and clicking the content type you want to modify.
- Use the technique shown in this page to hide the 4 fields in your newform.aspx.