Should I stop using Introjucer, and start using Projucer instead?
For new projects we will use Projucer, but are there any plans to discontinue the Introjucer, so that I should start using this for old projects as well?
Up to you.
We'll keep the introjucer open-source, and the two apps will always share a common codebase. If you'd prefer to build it yourself from source so you can hack it or get the very latest changes, then that's fine. Or just use the projucer if you want a pre-built binary.