Hmm. Well I just called them back and spoke to two different people, and not to rain on your parade but I think you're going to have a problem with that.
First of all, the 600/1000 tariff doesn't have unlimited data built in. Secondly, they absolutely point-blank wouldn't budge on the 'crappy tariff' - in fact they didn't actually know the details but I said to them "If you put me down for that tariff on that phone now and if, like you're saying to me, details become clearer nearer the time to my detriment, will you honour the deal?" and they said - "Well we're not making a deal with you, because it's too far away. Can't be done, sorry". I told them about your deal and both operators gasped and said "Wow - well that's not going to happen. That's a mistake. We can't do that."
Or words to that effect.
So I'm confused how you managed it but hey, I guess time will tell! Maybe you're the luckiest man in the world but something tells me it won't work out that way...
I got a similar response, but good luck to you eenu.