Comment 5 for bug 638584

That btree happens to work here is just getting lucky with the format not changing across these particular versions, not because it's somehow inherently "more compatible" than hashes. Btree is versioned just like hash is and can change incompatibly in any new BDB version.

WONTFIX - rpm might switch to btree by default for other reasons (such as potentially better performance) at some point but not because of false hopes of better compatibility. Like Jeff points out, there are numerous things to take care of besides just changing the default configuration, and while a future rpm version might be able to deal with on-the-fly btree/ht detection/conversion, there's little chance that such code would end up in existing RHEL and even less chance for EOL Fedora version.

Of course you're free to configure your own systems and chroots to use btree instead of hash while the luck with compatibility lasts.