I'd agree for an MMO, which can be quite complex server-wise. But most "online single player" would be quite easy to modify.

I'm a software developer who worked with asynchronous online systems.
A simple disk caching system could replace any uploaded data, and any online call can be written to work with cached data with a few line of code. Heck, on some frameworks you could write a simple middleware to make it work without changing a line of the original code.
I could do it on such game in less than a week on a language I don't know, and probably a day or two on one I know about.