I'm sure they feel that full marriage rights are preferable.

Full marriage rights are no different to Civil Partnership rights.
In terms of rights, nothing has been gained.

"All changed, changed utterly" is merely the right for gay couples to use the word married.
The meaning of one word has been altered.
That is all.

I know they do feel that is "preferable."
I know two couples who have made the change (my sister in law, our Methodist minister and their partners).

It is surprising how much emotional baggage is involved in obtaining just a word, and one which is going out of fashion generally anyway.