Have you checked the UI version of the pages?
You can manually change the UI version from 3 to 4 in PowerShell. This is done in the same way as on an
SPWeb, however the forms will be
SPFile objects in the Forms library.
Manage Visual Upgrade (SharePoint Server 2010)
To rectify this you have a few options:
If the forms were custom and you'd like to retain that look:
1) Open each form in SPD and edit until the form is back to how you wanted it.
If you'd like to revert to the default SharePoint forms:
2) In SPD, navigate to the site, click All Files, navigate to your list. If there is a blue/white exclamation point on the file icon you can right click and choose "Reset to site definition."
3) In the browser, navigate to the list > List Settings (in the ribbon under the List tab) > Form Settings > check "Use the default SharePoint form"