Is that a drop-in replacement for the DS1225Y? Have they been available this whole time?
I believe so, it's the Maxim part and you need to go for the "AD" suffix which has the same supply voltage spec as the original "Y" (5v ± 10%); anything faster than 200ns should be OK.
What do you think of using a FRAM instead?
It's worth reading the TekScopes mailing list archived postings (and, I believe, also threads passim on eevblog) about this. The problem with the FRAM is that there's no protection from spurious writes on power up/down as the DS1225 handles this internally. the result is that quite a few people have reported corruption of cal data when using an FRAM.
I originally bought an FRAM to put in the 2445B that I repaired recently but having read through the old posts I decided to go for the Dallas chip instead.