If we are arriving Gare de Lyon via TGV from Avignon and then transferring to RER line D at the station, do we need to buy new tickets before we enter the RER line or are we covered until we exit the metro (which would be at Chatelet Les Halles)? Thank you!
You need a ticket for the RER and Metro - the TGV ticket does not come with one. One ticket is good to transfer between RER and Metro, if that's what you mean.
The ride you describe is a regular metro ride. You do need a ticket for that, which you can buy in the metro, but it is all in zone 1.
If you don't want to queue for tickets, you can buy them on board the TGV at the bar. They're a bit more expensive but saves a potentially long queue upon arrival (rarely that long, but annoying after a long trip).
Also, from gare de Lyon to Chatelet les Halles, if Chatelet is your destination, you should just take metro line 14 which is more frequent. Same tickets.
Great tip, thank you.