[XeTeX] Change font metric?

[Thierry]

I would like to know if it is possible to change the font metric of a particular character in xetex.

The reason is the following.
TeX uses the unicode character 
U+2019 ’ GUILLEMET-APOSTROPHE
for apostrophes, as in (sorry, french example!) 
L’avant
In the tex file, I have
L'avant
which uses the unicode character
U+0027 ' APOSTROPHE

I have noticed that some fonts I tested with xetex does not have a sufficient space before and after the character U+2019.
This gives very poor results.

I would like to adjust the font metric of this particular character.
Is it possible?
An other possibility could be to change the U+2019 character to the U+0027 character, which seems to have a better font metric. But french typographic rules prefer the U+2019 character for apostrophes.
I have tested these characters in an ordinary word processor (TextEdit on my Mac): this is indeed a problem concerning the font metric.
In my case, the font is Minion Pro. With Adobe Garamond Pro, it is OK.

Thanks for any advices.
Thierry