Why can't one change the odds and stake at the same time?

I think there is a reason but .. yes I agree with you.
The reason they might say they can't is that way a modified bet works isn't a "replace with new bet" it's a more complex thing that has to take into account the original bet, and what might've happened between what you see and what has happened since etc, and to some extent allows keeping your place in the bet queue. They treat modifying a bet quite differently to making a new bet. It's one of the things betfair does very well (though I knock them sometimes ;)) and better than betd**.

But yes if you are COMPLETELY changing the bet and ditching the old bet, I know what you mean. It's annoyed me in the past. I think in the web site they could relatively easily modify an obviously new bet to the corrrect thing.