I'm of the opinion that it's your bike and you should be able to do whatever you wish with it. It's a good idea to preserve a bike by not doing anything irrevocable and keeping all the original parts but in the end it is just a bike, a machine -and machines are made to be used -not put on an altar and worshiped.
Start by acquiring decent tools and getting a good book like Glenn's and doing research online so you know what you are doing and don't mistakenly destroy stuff. But if one decides to modify something irrevocably and goes into it eyes wide open about the repercussions regarding collectibility and future worth then just do it.
Almost all of these bikes are not THAT rare or particularly special regardless of what many here might say. The factories made millions of old bikes during the 20th century and most of them were abused and used up anyway by consumers without any thought for future generations of C&V colletors. Tons of scrap was melted down when bikes ended up in dumpsters. What's the big deal if someone decides to modify an older bike? As long as it is given a new life I see absolutely no harm in it.