If your network doesn't do AQM that supports ECN marking, then the only appropriate thing to do is pretend ECN doesn't exist. There's no reason to tamper with those bits or even observe them unless you are going to use them to signal that you are experiencing congestion.

The current situation for the other six DiffServ bits is different. The lack of standardized meaning for them means it's common practice to alter or remove the prioritization if you don't agree with how your neighbor networks use those bits.

that would be a middlebox thing, not a BGP peer thing.