I own a few repair books and spoke with a couple of the local mechanics at the bike co-op and all of them tell me that I have to remove the crank in order to r&r the small chainwheel. But, I looked at the chainwheels and the crank and it looks like I can remove the CW's without pulling the crank... I kept looking and looking and I was convinced that I didn't need to remove the crank in order to change the CW's. But... I'm a rookie working on bikes... What do I know... So... I bought a crank puller, I bought the chainwheel nut wrench... and so I now have all the tools just in case I'm wrong and need to pull the crank.
Well lo and behold... I didn't need that chainwheel special wrench as there was no problem loosening the CW nuts & bolts, however, the wrench was only $3 and I'm sure I'll need it on some of my other bikes. And... since I didn't need to pull the crank... I didn't need the crank puller and this turned into a VERY simple job needing only the 5mm allen wrench to do the job.
No doubt in many instances the crank might need to be removed, however, I just want to tell people, that this is not always the case and I'd probably guess that a MAJORITY of cases, the crank will not need to be removed when replacing a chainwheel on a 2 CW road bike.
My case, I have a 52t/42t and wanted to replace the 42t with a 39t and that was a piece of cake.