Even if Godzilla vs. Kong was still set to open in March, it would still be in post-production at this time so editing out certain scenes or lines of dialogue wouldn't be something that would require an eight-month delay. That's why I fear this delay might indicate more significant retooling, like completely reshooting existing scenes just to give the actors different dialogue. What good did it do Justice League to give the characters more quips and one-liners than there were in Batman v Superman? It didn't make a difference.
The most common complaints about Godzilla: King of the Monsters from people who saw the film can be boiled down to 1) "I thought the action scenes were too dark & rainy," 2) "I didn't care about the human story," and 3) "I didn't laugh at the humor."
I can see complaint #1 being a major change that WB might be trying to make with GvsK. If a lot of the monster fight scenes are set at night and/or in heavy weather, that's something they might be actively trying to change in response to G:KotM's backlash. Regardless of whether that might make for better onscreen spectacle, it definitely wouldn't be a cheap or easy alteration. They would need to reshoot the live-action plates for those sequences, possibly reshoot scenes with some of the cast, and redo much of the digital effects work they've already done.
But the biggest complaint regarding G:KotM didn't come from people who saw it. It came from people who decided not to go see it because they didn't care about seeing a new Godzilla movie. As unpleasant as it is to admit it, Godzilla really is the root of the problem with this upcoming crossover in terms of WB's ability to sell it to the public.
Most people are not Godzilla fans, not in the dedicated "I post on Godzilla message boards" sense and not even in the casual "I'll watch a big Godzilla blockbuster if Hollywood offers it" sense. The 2014 film was able to draw a lot of curious lookie-loos on opening weekend because it was the first Hollywood Godzilla film in a generation and thus the first Godzilla film that many people ever actually watched. It did not get audiences jazzed about the prospect of a sequel and it was just as frontloaded as GINO was in 1998. WB's hope was that the web-based nerd criticisms of G'14 reflected the general audience sentiment, that audiences really wanted to see more of Godzilla and the sequel just needed to be action-heavy, but that was not the case. G:KotM was as frontloaded as G'14 and GINO'98. In the real world, most people just aren't interested in Godzilla as a character.
That's why, even more than changing the action sequences to clear daylight, I wonder if this eight-month delay is about WB trying to retool Godzilla's role in the story. G:KotM was meant to get audiences cheering for Godzilla and GvsK was probably made with that in mind. The version of GvsK that wrapped shooting back in April probably portrays Godzilla in a heroic light and the story probably ends with the door left wide open for further Godzilla adventures. WB is in a very tricky situation trying to sell Godzilla vs. Kong to a public that has shown rapidly declining interest in Godzilla, so I wonder if they might be hedging their bets on audience interest in Kong and retooling the movie accordingly.