so this is my "worked in aws" informed take, but you absolutely do not force default behavior change on anyone unless it is absolutely necessary, and advertising http/3 isn't necessary.
AWS had a similar oopsie with their Load Balancer: introduced a new flag to strip some headers, enabled it by default for all existing public ALBs, and the effect was that all custom headers with an underscore in the name (“x-access_token”) started getting stripped. Took down a few APIs https://forums.aws.amazon.com/thread.jspa?messageID=923182