Please elaborate on your understanding of "other differences", plus the range of Euro appliances involved (and their specs) that are under consideration.

It might also be useful to make up a small sign saying:
"If It Was Easy Everyone Would Do It"

and place it where it will be a constant reminder.

My neighbor (who is Swiss) put a 240V receptacle in his kitchen for similar reasons. Yes you can indeed put a 6-20R in. Yes, you need to switch to a 20A breaker. You may need to do a little extra work to deal with the size of the wire if the receptacle can't take something that large (the breakers usually aren't a problem).

A GFCI is not strictly required here but frankly since you're replacing the breaker anyhow, putting a GFCI breaker there would be a VERY good idea.

Replacing a regular outlet with this new one may cause a code violation due to the required spacing of the countertop outlets.
Adding this as a new outlet should be fine.

thanks for the responses. Good point about the countertop spacing of regular 110V outlets. In that case, I will probably just add the 220V as a new outlet. Thinking about it more, I guess I should probably blank off the range outlet, since I will have downgraded the breaker on that circuit to 20A. Is is OK from a code perspective NOT to have a functional range outlet?

I don't know about building code - but I would worry about future resale value. Most people aren't going to be too happy if they have to hire an electrician before they can hook up their standard USian type stove or cooktop.

You stated you have gas. You do not need a 220v range outlet in this case. It is not required by code.