Hi Rosetta,
unfortunately BluRay sup are not 100% compliant with DVD sup. I may suggest you following (not so confortable) steps:
- convert BD sup to sub/idx using BDSup2Sup
- convert sub/idx to DVD sup using Sub2Sup or VSConv (the latter is included with A2D package)
- check the resulting sup with SubtitleCreator (with A2D package as well)
Any other suggestion is welcome
Bye