Since EQ version 18 you can create quotations using more than 1 furniture catalogue. A quote heading contains a collection of furniture settings including the catalogue name (often the manufacturer), ranges and other colour and finish options including drawerboxes and handles, depending on the catalogue. See this article on Creating quotes with more than 1 furniture catalogue.
Once you have added furniture products onto a quotation you can change or replace a product for another product in the same heading but not for a product code that is not available in a different catalogue.
For example: A drawerline base unit BD10 may exist in 1 catalogue but the similar unit may have a different product code in a different catalogue.
If you use the Heading column to change the product:
Or you right-click the product code and select Change to Alternative Heading...:
And you select a heading which uses a different catalogue you will be informed that you cannot make the change. The product code does not exist in that catalogue and cannot be changed.
Instead, you should Add or Insert a new product directly into the heading you need.
You can also delete the existing product.
You should click the Add button and select your heading to go through the product search screen:
Or you can right-click a product code and select Add New Item or Add New Item [Insert Before]
Both will allow you to search and locate the desired product and add to the quote in the relevant heading.