I went through this thread but since the info I found was quite old, I'm still not sure what the answe is.
Planning to buy a ticket with our son as Lap child. The trip has 2 segments:
1) Europe-USA in V fare on United
2) USA-Central America on V fare with Copa
I was planning to buy the lap child ticket immediately (with my ticket) and try a GPU on the 1st segment and a RPU on the 2nd.
I'll pay the 10% for our son lap child upfront but will UA charges me extra if any of the upgrade clears?