I just responded to someone who asked, "If one were to move forward on a complete and total career change, and one has various online profiles such as Twitter, LinkedIn, etc. -- what should one do with those now 'old' profiles? Given that there would be virtually no one in the old contact list that would be interested in the new career direction, and the profiles are used for professional reasons only (i.e., friends would have no trouble finding you or already know about the change), is it safe/wise to simply delete the old profiles? Is there a waiting period that is recommended? Six months? A year? Two?"
My reply: I recommend revising/editing those profiles to make them relevant to the new career; I wouldn't delete them. Here's my thinking: I view any career change as being "a new true story" about oneself, including one's past experience. It may take some thought, and then a little work, to re-vision past experience as prologue to to the current reality, but it's well worth thinking through what is still relevant going forward. As just one example, I had a client once who was moving from being a veterinary tech to being a computer programmer. I agreed that folks in IT were unlikely to be interested in her ability to give a cat a shot, but then we talked about her ability to make quick decisions in a criss situtation and to do that while highly emotional people were yelling at her. Those are what we call transferable skills, which are valuable in almost any career and are skills you carry with you wherever you go.
The so-called "soft" skills are the ones most likely to be transferable to a totally different field: judgment, communication, mentoring, reading between the lines, seeing the big picture, and on and on. On LinkedIn, for example, you can go through your profile and edit past experience to highlight parts of your background which apply to your new career and downplay or omit the more specific skills that used to be your selling points. And of course change your headline & summary to be relevant to your new field. That way your new contacts and colleagues will get an up-to-date message about the value you bring to them.
Remember also that some percentage of your old professional contacts are also multi-dimensional folks, so updating your profile may give them just the right stimulus to introduce you to people in their circle of influence whom you might want to know - their spouses, siblings, neighbors, friends, old college roommates - who are in your new field. You'll never know about those connections unless you get the word out through your existing (old) professional network, and even if it's only 10%, that's worth it.
Twitter can be a different deal, especially if your old handle is specific to your old profession. But on Twitter you can easily just start a new stream, with a new handle, and do some cross-posts from the old one over time to bring over whatever percentage of folks care to follow the "new you." There's certainly no harm in tweeting about your transition on both streams, and doing that may help you find-tune your new messaging.
Bottom line: Think about the message you want to convey now - your new true story - but let both your old and new professional circles know enough about you to give them a chance to think, "Ooh, So and So is even more interesting than I thought. I should pay more attention to her."