With climate change, we're looking at drastic coastline changes over the next 100 years. 100 years is extremely short for cities - it probably takes about that to go from being a town to being a city, if you have excellent growth rates.
To move a city in 100 years is probably flat-out impossible, just for cost reasons alone. Most cities are hard-pressed to pay for maintenance of roads and essential services, and service their already crushing debt levels. City governments are not going to manage it, which means the citizens of the cities are going to shoulder the costs. We're going to see large-scale financial hardship in coastal cities going forward, and multiple cities going bankrupt and into decline.
[1] https://ocean.si.edu/through-time/ancient-seas/sea-level-ris...
Venice will likely have to take similarly-drastic measures, and they're a lot harder to take given that modern-day Venice is much bigger than 1850's-1860's Sacramento.
Scale matters a lot.
If humans weren't polluting the atmosphere, the climate would still be changing. The only difference is time scale and possibly the temperature delta.
This is why building cities right up against the shoreline isn't a very good idea in the long term. And, no, you can have ports without building metropolises right next to the water. Someone's going to say that I'm a "climate denier" which is totally false because I believe in anthropogenic global warming.