Upgrade Debian testing to stable at Sunday

[Jason Voorhees <jvoorhees1@gmail.com>]

Hi people:

I got a business with one of my customers to install him a
Firewall+Proxy+VPN running Debian this Saturdary 14th. I was thinking
about installing Debian Etch because I din't know that Lenny was so
close to come.
Now I think I would like to install Debian Lenny but maybe at the time
I will do the installation (at 14:00 GMT) maybe Debian isn't released
yet or I won't have enough time to download the ISO image.

If I install Debian Testing at Saturday and then I upgrade to Stable
at Sunday... will I get any kind of problem? would you suggest to
install debian testing and then upgrade to stable some hours (or days)
ago?

Thanks