Last epoch switch between faction possible ?

Hello LE gamers !

When I came at the end of the story I had the choice between the 2 possible factions (merchant vs grinder), it was written "watch out, your choice cannot be changed once a selection has been made".
I choose for grinder and I'm at rank 9 by now.

But strangly I still have the option to join the other faction (merchant), if I do so, will this mean I will lose all my ranks in grinder ?

Do I have to create another character if I want to sell something in the market ?

Thanks for precision and help.