[XeTeX] Change font metric?

[Thierry]

> This change is done via the Mapping=tex-text option. You
> can either not use this for your defective fonts or change
> it according to your defective fonts' needs. Then it'll be
> best to have an "adobe-text" or such option that the working
> fonts can continue to work.
> 

I have tried without the Mapping=tex-text option and indeed the apostrophe from the source is used into the pdf, and the metric is better (but the glyph is not so beautiful!). This can be a solution.
What I don't understand is the following. Using pdflatex, this change takes place automatically. With xetex, this is a kind of option. Is there a way to create a personalization of what is done and not?
You mention an "adobe-text" option. How could I produce this? I don't know where the rules applied by the Mapping=tex-text option are defined. IS there a way to do the same with pdflatex? (I use it for some projects).

Concerning other propositions which have been made, I have some remarks:
- one of them is to manually change the input apostrophe into the output one. This doesn't solve the metric problem. 
>From a practical point of view, this find and replace procedure is not very convenient. I have remember to use it each time I change the text. I prefer to let (xe)latex do it for me!

- the trick
\catcode"2019=\active
\def’{\leavevmode \kern 1sp \hbox{'}}
works, except for the practical problem mentioned above. Is it possible to activate the U+0027 character to do the same? Will this produce conflicts elsewhere (in math mode for instance)?

- I could change the metric in the font installed on my computer, using for instance "FontForge", but I really don't know how to do that! This software is not easy to understand. I only know how to use it to change the encoding of fonts files and save them in pfb to install them in my tex tree (old procedure before xetex).

Thanks a lot
Thierry