The 650BD/651BD/751BD/752BD support the following video formats; AVI, MKV, MPEG2, MPEG2 HD, MPEG4, MPEG4 AVC, VC-1, XviD, VCD, AVCHD, JPEG
However we would advise you not to convert all your files. You should copy them and convert only the copies.
It is obviously essential to always have a backup in case of problems and we are not liable for consequential damage.
In terms of support we keep archived copies of all firmware so if there were some major reason why the machine could not support AVI or MKV in the future we could make available to you an earlier firmware (such as the present one) that does support them.
You should also test a few of your files on the machine to make sure you are happy with the performance and file support.
This is a very fast moving area and we have seen changes in BD disc authoring for instance already that make guaranteeing any particular file or format playability difficult.