I'd better get a hard hat, because I think Hammond has a point.
Until negotiations start, I don't see how anybody can make any promises. From a practical point of view, it seems unlikely that the UK will swap its EU immigrants with UK ex-pats. The logistics and expense of the whole exercise don't bear thinking about.
However, if the UK makes an absolute promise to EU citizens that they can stay in the UK indefinitely, what happens if the EU turns round and says it doesn't want UK ex-pats and/or will stop healthcare arrangements, etc? The UK would be in an impossible bargaining position. We could end up with an extra couple of million ex-pats not able to afford property, many needing NHS care plus the immigrants who are already here OR ex-pats would end up high and dry in foreign countries.
I really hope it doesn't come to that and I hope that when negotiations start, this will be one of the first issues to be thrashed out, but until it's been formally agreed, I'm afraid I think it's a bit silly to make any promises. In the same way, the promises Leave politicians have made and are still making (Don't worry, dear, it will be alright) are premature and shouldn't be believed.