In Thor #169 Galactus recounts his origin to Thor and mentions that the Watcher (Uatu?) observed him on his ship after everyone else on his ship perished after they crashed into a sun (because a plague destroyed their world Taa). Galan at this time was infected with cosmic power but hadn't woken up yet and become Galactus. The Thor story doesn't even say Galactus survived the death of his universe...that must have been added later.
So does that mean the Watchers existed in Galan's iteration of the multiverse or did we get a retcon later that established this Watcher only observed Galan after the universe was reborn (meaning the Watchers probably did not exist in Galan's original universe).
It was retconned. Initially, Galactus did not survive from the previous universe either. He flew through a nebula or some other event and gained power that way. The survivor of a previous universe was revealed later.