AFAIK, the main difference between the 100 and 200 is one of max gross weight, and the associated structural changes. A lot of the other changes mentioned in other posts are more in the nature of incremental improvements/changes to equipment fit that are normal in any production run -- and which are often retrofitted to earlier aircraft as well.
These days, the changes that resulted in the 200 might have only resulted in it being called 100ER, but that's today.
But of course today, they'd never have a -100 to begin with. They'd start at least at 200. And the baseline model would already be an ER. So if the 747 was a new model, what we know as the 100 would probably be the 200ER, and what we know as the 200 would be the 200ERX or some such.