Welcome to Premium :-)
– You’ll be reminded both via email and via a notification in the application when your Premium is about to expire. We will not charge you automatically though. If you let it expire, nothing ‘bad’ really happens. You’ll have full access to all your encrypted files (but new or updated files will use 128-bit) and if you use the password manager, you can read all but not update.
– That’s a feature that’s not yet implemented, but we’ll have a ‘upgrade 128 to 256’ bit function in the not too far distant future. I’ve added an issue in our tracker for this: https://bitbucket.org/axantum/axcrypt-net/issues/202/function-to-convert-128-bit-to-256-bit . Right now, you’ll have to decrypt and re-encrypt (that’s actually what needs to be done anyway, but we can automate it).