It can be done. Really depends upon the exact bike-model in question and how it was design and built. A lot of 650b bikes started life as 700c and can be converted back easily. Post pictures of your bike with measurements of the following areas:

1. fork-crown to tyre-clearance
2. front-brake, side view of pad-position
3. front-tyre to downtube clearance
4. rear brake-bridge to tyre-clearance
5. rear-brake, side view of pad-position
6. rear-tyre to seattube clearance

If you've got 19mm of clearance and adjustment in these areas, it is a simple swap to 700c wheels. Otherwise, you may need to do some cutting & re-brazing/welding. Similar to converting 700c road-bike to cyclocross or a 26" MTB into a touring-bike (adding/removing cantilever posts), maybe change forks. Not that daunting, I've done quite a few.