In the lounges it has been a change to the Schweppes 1783 premium product not a direct switch back to what was offered before Fevertree.
I will leave others to judge which is the best product. I am personally a big fan of the Fentimans range though.
Pure speculation but I suspect Coca-Cola (Who I believe own the Schweppes brand in the UK) have offerred them a very good deal to BA as they have presumably lost significant market share to Fevertree and others over the last few years and want to get their 1783 range off the ground.