Unable to mount / in read - write mode

[Popof Popof]

Hi,
I recently tried to update my FreeBSD 6.0 to FreeBSD 7.0.
I don't know where but I made a mistake and I am always booting on the 6.0
kernel.
The problem is that I have an error during boot process:

mount option <rw> is unknown
> mount: /dev/ad0s2a : Invalid argument
> Mounting root filesystem rw failed, startup aborted
> Boot interrupted


Its seems that tools have correctly upgraded (man mount let me see that I
use the FreeBSD 7 version of mount) but not the kernel.

Does someone has an idea to allow me to use my filesystem in read write mode
?