I’ve experienced this exact scenario. I joined the Ambassador program after booking accommodation with the Intercontinental in London but before my stay.
Upon arrival at the hotel, I was given the upgrade and enjoyed all other Ambassador benefits, including fresh fruit, chocolates and wine in my room.
Just be aware that if you are hoping to redeem the Ambassador Free Weekend Night Certificate for your stay, then technically you should already be an Ambassador before making your booking.
However, there is a bit of a work around. If you go to this link on IHG’s website, you can select "Ambassador Certificate’ from the Rate Preference drop-down menu, input your city and travel dates (make sure you cover a Friday or Saturday) and search for availability on that rate. This will allow you to complete an Ambassador Certificate booking before you have received your Certificate or joined the Ambassador program. You will then just need to present your Ambassador Certificate at check-in.
Hope this helps,