I have a Marantz 9200 (Marantz version of the 6000) that is a US spec model. I have updated the firmware to the European version, when the Euro version was a newer firmware release than the US version and solved a specific problem. There were no ill effects.

In your case, I don't know what you'd gain by doing this, however. The 6000 is out of production, and the firmware is fully mature in both the US and Euro versions. Yes, the code databases are different, but changing your firmware won't change these codes. There are other differences in the remotes as well. RF frequencies for instance. Again this won't change via the firmware.

In short, while you SHOULD be able to load the Euro firmware, I see no advantage to doing so.