Upgrading reward ticket at airport with miles.

If you purchase an economy ticket for 50,000 miles and pay the taxes can you then upgrade that ticket to premium or upper using the extra miles required if there is availability at the airport on your day of departure.
We want to use some miles but there are no premium or upper available and I know if you have a qualifying ticket you can do this but I was not sure if you could do the same with a miles only ticket.
If anyone knows the answer, I would be most grateful.
Many thanks,
Edit to title of thread by mod for clarity....Nick
We want to use some miles but there are no premium or upper available and I know if you have a qualifying ticket you can do this but I was not sure if you could do the same with a miles only ticket.
If anyone knows the answer, I would be most grateful.
Many thanks,
Edit to title of thread by mod for clarity....Nick