Q:
All PDFA levels (1,2,3) have an A and B option. Is there a major difference between A and B?
A:
Level B is less restrictive, and simply ensures that the visual appearance of the PDF is stable/reliable.
Level A has all the same requirements as B, but adds more requirements for semantic information, such as unicode mappings for accurate text extraction, and other metadata.
From the ISO specification
Level B conformance
conformance level encompassing the requirements of this part of ISO 19005 regarding the visual appearance of electronic documents, but neither their structural or semantic properties nor the requirement that all text have Unicode equivalents.
If the content for Level A is not present in the PDF, it is typically not something that can be added automatically, but would require manual work.