If you go to the Alsa website and plug in Madrid -Ponferrada, you will see that when the bus schedule comes up, the designation for the 10 am bus from the airport is “Superflex” and underneath it says — changes and cancellations permitted. I have had good luck with Alsa and their ticketing but have never had to change or cancel so I can’t give you any first hand experience.
I suppose the other option is to just assume that the bus will have seats, which is extremely likely. It starts at the airport before going down to two other in-town bus stations (Estación Sur and Moncloa) so you will be ahead of any walk-ons from those stations.
Do you know which terminal you arrive in? The Alsa bus leaves from T4, which is where Iberia and all its One World partners land. If you land in the other terminals,you will have to catch an intra-airport bus over to T4. The buses are very efficient and leave every ten minutes, but it will add another 15-20 minutes to your journey.
And who knows what immigration and the health form check will look like when you arrive!
Good luck and buen camino. Are you by any chance walking the Camino Invierno from Ponferrada?