RE: Are Fields removed from Page after Upgrade, is this true?
There are two parts of the story, I suppose.
One part is that the user interface is customizable, and users may setup their page the way they like it (e.g., hide some fields or reveal the other fields, change size and order of the fields and factboxes). This is stored as their own settings and next time they open NAV, they see their unique setup. However when you have to modify a page or table behind the page, in some cases you need to re-compile it and it would lead to the fact that all the settings on the user interface would be lost (e.g., revealed fields will disappear and other will appear back). Users would need to perform setup once more.
The other part is that rolling up an update would not usually modify additional customized functionality, as the range of the objects of customized functionality is usually different from those used in standard updates (e.g., you have added a field in a table, and most likely it will not be affected by the update). However, if you had modified the objects of standard functionality, or your additional objects are referred to standard objects that are changed by the update, you indeed might have a risk of loosing the information, as a conflict within the update would appear.
If your partner will take care of the conflicts properly, it should not be harmful or should not lead to disappearing of the objects or data. But, if they will not take care properly of the conflicts, some damage might happen.