Upgrade Debian 7 to Debian 9 - I have new OS on microSD, what next?

I need to upgrade my BBB from Debian 7 to Debian 9. I'm just going to
install Debian 9 and then re-install my customisations.

So, I have a Debian 9 image on a microSD card and have put that into
my Debian 7 BBB. It boots up Debian 7 and the new Debian 9 is on a
mounted filesystem (well, two actually):-

    /dev/mmcblk1p1 on /boot/uboot type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
    /dev/mmcblk0p1 on /media/rootfs type ext4 (rw,nosuid,nodev,relatime,data=ordered,uhelper=udisks)

How do I get the new Debian to install onto the existing memory on my
BBB?

When I plugged the microSD card into other (non-customised) BBBs I
have it *automatically* installed itself but it hasn't done this on
the one wher I need it. Is there something that needs adding or
uncommenting in a boot script somewhere?

Thanks for any advice.

I've been hunting around a bit trying to sort this for myself, not
found the solution yet but there are some more clues, maybe.

When I say 'existing memory' I mean the eMMC, it's a 4Gb one so can
cope with Debian 9 OK.

The /dev/mmcblk1p1 is the eMMC boot partition, Debian 9 is on the
/dev/mmcblk0p1 mount.

Most of the web searches I have done take me to uncommenting a line in
/boot/uEnv.txt. I don't have a /boot/uEnv.txt, only a /boot/uboot/uEnv.txt.
However my uEnv.txt doesn't have a line anything like the ones that
are suppoesed to be uncommented so I'm still a bit stuck.

As above, any help/suggestions would be most welcome.

On Mon, 2 Apr 2018 13:56:23 +0100, Chris Green
<cl@isbd.net> declaimed the following:

When I plugged the microSD card into other (non-customised) BBBs I
have it *automatically* installed itself but it hasn't done this on
the one wher I need it. Is there something that needs adding or
uncommenting in a boot script somewhere?

  Off-hand... Try holding down the "boot" button when applying power. It
would seem this unit is old enough to not have the first stage boot check
for executable SD card images.

  I'm presuming the SD card is configured as a "flasher" since you state
others have updated from it.