Customers who have a commercial license for iText 5 Core, and who are current support and maintenance subscribers can upgrade to iText 7 Core for no additional cost. Additionally, customers who have an XFA Worker license can get a free upgrade to the iText 7 replacement, pdfXFA.
Instead of having everything in one giant jar (or DLL), we have split iText into several independent parts. You pay only for the parts you use, and your application only needs to load those parts that are actually being used.
iText 7 is a complete re-design of iText 5, it includes all the functionality of iText 5, but in a way that provides a clear and consistent API. iText 7 features a new rendering framework and several changes in the document model. iText 7 puts more emphasis on the separation of the high-level document model and the low-level PDF operators.
Note that these changes and improvements mean you will not be able to simply replace dependencies and expect your code to work. Some migration effort on your part is to be expected.