First BA trip coming up, so have no status. Have a PHL to LHR then a connection to Paris, originally in CW and CE. Paid for seat selection for CW but held off on the CE flight as thought it'd be a waste. Later upgraded the first leg to First Class, and was surprised to see that I was then able to select my CE seat on the connection for free.

I know free selection comes with F flights, but is this normal for that benefit to extend to CE connections on the PNR? Or just a lucky glitch?