We switch from 5.2 to 5.3 because we've done a major internal upgrade with respect to security-related functionality (encryption and digital signatures). The previous version uses version 1.46 of the third party library "BouncyCastle". We've now upgraded to BouncyCastle 1.47.
As the API of BouncyCastle has drastically been changed, we had to rewrite parts of the internal iText code. If you upgrade to iText 5.3.x, you'll also need to upgrade to BouncyCastle 1.47. Depending on how heavily you depend on BouncyCastle, you may have to change some of your own (BouncyCastle-related) code for digital signing.