I believe Anne Daunted is on the right track here, but one other thing to consider is that Spam is inherently promotional in nature. The gravamen isn't the mention of the affiliated product or service (it is not a playground "forbidden word" game, where saying the name of the thing makes you "lose"), it is the promotional intent, or at least the potential for it, that makes the post so concerning that it deserves red flag treatment. If there was no affiliation at the time of the post, there was likely no significant pressure on the part of the poster to whitewash or otherwise paint a glowing portrayal of the product. It's like a professional "conflict of interest" - if there was no conflict at the time you made the decision to perform a professional act, no later role conflict can go back and improperly influence that unless you have also gained time-travel technology (in which case please share it with us). Consider: You're a physician, and occasionally you prescribe one of those new, really expensive designer drugs because you think they would be really effective for one or two of your patients. That's not unethical, it's your professional duty and judgment to do so. Now, one day, the manufacturer gives you a huge grant. Now things are decidedly different, and you should consult with a professional ethicist or at least make full and complete disclosure to any patients you are currently seeing before prescribing this drug again, because your judgment is at least potentially clouded by visions of green.
The pressure on the poster to post truly promotional content rather than informational or helpful content changes when the affiliation happens, so I would support a rule that only later posts truly require disclosure. Non-trivial or disruptive edits of prior posts could also trigger a disclosure requirement, depending on the substantiality and/or frequency (e.g. a quick spelling fix to one or two posts probably would not, a major enhancement probably would, and repeated, minor serial edits for the purpose of bumping the posts to gain more exposure for the product or service definitely would, and would probably be discouraged through suspensions).