I agree, Adam. I think the removal of the filter as well as the ability for anonymous leads to process through smart campaigns is going to cause some headaches. There will inevitably be system 'racing' issues. For example, if an anonymous lead comes in through a clicked PPC link, later through a social link and lastly via a content syndication link, but only becomes known after multiple visits via a conversion resulting from content syndication, we no longer have the ability to correctly attribute that lead's source to the channel that originally drove them to the site (PPC in this case). We will have the ability to track the source by which the lead became known, but I know there is a lot of debate about whether source is the channel that drove a lead to engage initially (even if they didn't convert) or if it's the channel that drove a lead through conversion. In this scenario we'll be inconsistently setting source because the source will likely be set on conversion and since the field is locked down, even though leads qualify for campaigns once they become known, we've already updated that value which can now not be overwritten via the campaigns the anonymous leads would have qualified for upon visit to indicate the correct original source. Also, for anyone who has been using source to track original visits, as you have, will now be reporting inconsistently as their referential data is looking at original visits and sources set after 2/5 will be looking at conversion source. I'm looking through the resources in Gregoire's reply, but if anyone finds a workaround let me know!
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