Hmm. Some of the older URC remotes were sometimes a little bit "off" in how they reproduced codes... and it might be enough to throw off the MX-450's learning and result in them being unusable? Although, with that said, I've used an MX-700 as a donor remote for MX-500's before without issue.

Have you tried some of the tips as posted here: [Link: remotecentral.com]