I made this transition myself recently, and have been having the same questions about what my GTD practice means to me now. I adopted it around 2005, and it's seen me through several job and life changes. I don't have all the answers yet, but here are a couple key takeaways so far.
1 - Anchors. Getting ready to go to work, arriving at work, etc. These were all the rhythmic hammer beat of my days. After I left them I found that I had attached triggers to these for using the GTD system. Without these, we can feel unplugged and ungrounded. Pick yourself some new anchors. They may seem silly, but that's ok. You just need a new reliable timed set of events that you can use to trigger you to check your system, do weekly reviews, etc. "Leave work on Friday afternoon" is gone, but maybe "Saturday morning breakfast" is a great replacement!
2 - Time. Stop attaching tasks to time! I was already a firm believer in this for years, because modern life (especially in the tech world) creates a fools errand if you try to promise a task by a certain date. And nothing presses the self-fail button like moving yesterdays tasks to an already overloaded "today" list! Instead, use your GTD projects as containers for projects, interests, hobbies, commitments, volunteer work, etc. Then you either schedule a block of time on your calendar to work on it, or you just decide to do whatever you please on any given day instead. But either way, you do not schedule tasks! The only exceptions are things like "take out the garbage every Tuesday night", "replace furnace air filter ever six months", etc.
3 - Contexts are even more valuable than before. Things like "errands" remain about equally valuable, but I now find myself much more focused on a location or interest area than before. For instance, instead of thinking in terms of projects, "Record a new song", "Learn X new scale", etc., I now wake up and feel like playing "guitar" for a while. The Guitar context leads me to all the projects that I at some point decided would be valuable for me to pursue. This ensures I'm not just noodling and goofing off, but adding real value and satisfaction in my time spent on interests. I don't have evidence yet, but I believe this will be a key to make sure I don't wake up 20 years down the line and wonder wtf I've been doing all this time!
4 - I'm finding that my project support materials/software/services are FAR more valuable now than in the past, and far more than my projects/contexts list. I am mostly using ToDoIst now for nudges and reminders, and putting the bulk of what I work with and think about into other places, like Milanote. It's the analog of spending more time in the woodshop and less time talking with others about what you are going to do in the woodshop once you get there! I can now check my GTD lists maybe once a day, and do a check in and sweep, but otherwise I put all my time into the intrinsically rewarding things themselves.
5 - Don't rush the staged changes. It's cute how we all think we can step off the warp speed treadmill, take a weekend off to sleep in, and then in one sitting completely retool our GTD system (and life!)
We all do, so no worries. But I'm 9 months into this thing and feel I'm maybe... maybe on a good day, halfway to what I really need going forward.
Would love to hear more thoughts from anyone! There are so few of us in this position to learn from!
