Yes, if you download the code and change it, you can have a premium version as far as the local capabilities are concerned. It is entirely within the license agreement (GPL).
However, parts of the functionality is server based, and human based (password manager, direct support, key sharing, etc). These will of course not be available with a hacked version of AxCrypt.
We’re hoping that the free version delivers enough functionality to be really useful to very many, and that the Premium version delivers enough additional value to be good value for the money, for those who wish to support the project and/or want to have access to all those features with full support and maintenance from us.