I'm going to re-post the message I sent you in Private Messages about the same question:
Yeah, the switch to ECID can be scary. From my understanding, the way Adobe works (and looks at various pieces of information, your users will still have their s_vi cookie, and will then start to get an ECID when you implement it.
Adobe will continue to collect the s_vi information (which technically correlates to the Raw Data visid high and low), but will start stitching in the ECID identification... I don't know how Adobe maps the same user from multiple domains / multiple s_vids / multiple visid high/low together to be a single user.... Adobe really doesn't share how that deep engineering works.
When we made the transition, I don't recall having to do anything special.... that was about 10 years ago though, so my memory may be a bit fuzzy.
One thing I did do (and still do today), is store the ECID in a custom dimension. The Raw Data visid high/low is not stored in a format that matches the ECID, and at the time, the ECID wasn't a part of the Raw Data feeds (I think that may be included now... but to be safe).
I suppose if you are worried, you can test on your Dev or QA suite first, identifying yourself and maybe a few other people on multiple sites, then implement the ECID and look at what happens afterwards?
