Unfortunately not. You could invest heaps of time in coding an explorer-plugin to do that for you, but if the Content of the document is really encrypted, almost all bytes have to be rewritten.
Maybe you can change the Password via the C# API, that should be pretty forward then, but maybe the C# App has to open the doc in the Background (I don't really know, as my office-api-coding-time was quite a while ago) as well and you would have to code for each Office-Application by itself.