This is a message by Hank of URC posted back in 2017 on why mx450 new is not compatible with old mx-450 files:
This message is to make our dealers aware of some changes to URC’s popular MX-450 remote. This hand-programmed remote has been in the URC line for about 10 years and has remained a dealer favorite due to its great price and ability to be programmed without the use of a computer.
A few months ago URC was told that some of the raw parts needed to continue to make these units were no longer available and would need to be replaced with new parts that would require significant firmware changes to work. Although URC considered discontinuing the MX-450, or at least renaming it, we decided that its reputation was too strong and that we would simply make these “running changes” to this model.
Replacing the required parts and updating the firmware has uncovered incompatibilities between the older MX-450s (and their archived files) and the newer units. This inability to use older files on the newer remote was unavoidable and is not something that it is possible to fix. Unfortunately this will require any new remote to be programmed again and any older files will be unusable.
The good news is that the newer version of the MX-450 is still fully compatible with the MX-450 Archiver (its files can be saved and recalled at a later time).