I will have to agree with you that these should all be synonyms (damn IBM for not making up their mind what to call this thing), however as noted on the MSO post I have to also agree that it is much more commonly referred to as an AS/400 than it is an iSeries or an IBM i.
I have to wonder that if these posts are all retagged with [ibm-i] as you propose, whether people looking for support on these systems (who are most likely searching Google using the AS/400 keyword) will miss out on a great support resource because Google spidered the page with the [ibm-i] tag and not the [as400] tag they were probably looking for.
Ultimately, though it might be the right thing to do, I think it may actually harm people looking for support on these systems because they are simply searching for a keyword that search engines will not be indexing on Server Fault.
I know when I want something AS/400 related, I always search Google using a search with AS/400 in the query, because that is what most people call it, and to be honest finds the best/most relevant results.
Like I said, I'm all in favour of synonyming these tags, but I would rather see the [as400] tag as the "base" tag to which the others are synonymed to because of its popularity.
I still call our system an AS/400 because if I call it an iSeries I get some pretty vacant looks - people would think I'd totally flipped if I called it an IBM i!