What happens when an older version of zbackup tries to read a bundle compressed with lzo? Would it just fail because the compressed data does not look like lzma?

I propose to set the version to 2 in the header when the compression method is different from the original 'lzma', and otherwise use version 1. This way older zbackup versions would be able to decompress bundles created with the newer versions as long as they still use 'lzma', but in all other cases it would fail with the more informative 'incompatible version' message.