You don't, you run them in parallel to HTTP/3 and it shouldn't add any cost for you. The work is done in the browser (or any client) to fallback to whatever works.

Then, in a few years, when you notice that most of the traffic is done over QUIC, you still won't do anything as it has no significant cost anyway and helps the few % of people who haven't upgraded their network. And customers who can use it won't notice much as your website still works anyway, but your latency metrics should be better for those users.