Cfrtjryk

I’ve been wondering if "That is what….," and "This is what ….," in the following passage (taken from Fieldfish.com) can be used interchangeably.

Imagine you are an unmarried couple who have been trying to conceive for years. With the help of a well-established fertility clinic and donor sperm you undergo IVF treatment, and have your much desired child. In the course of the fertility process you are told both parents need to sign consent forms that once signed will confer on both the biological parent and the non-biological parent, the same rights of parentage without needing to go to court after the birth to get a declaration of parental responsibility, nor adoption orders. Then some months later the clinic calls you to tell you that due to an admin error, the forms were not completed correctly and the non-biological parent is not legally the child's parent, and probably the only solution is to go through the adoption process.

That is what happened to many couples in the UK who have had fertility treatment using donor sperm and eggs. This is what happened to a family in 2013 and it prompted the Human Fertilisation and Embryology Authority (the HFEA) to require all clinics to audit their cases to see whether there were any other failures by clinics of having failed to get the family to sign both consent form or, having lost or misfiled these legal consents.

“That” and “This” here seem to be almost the same.
It is my understanding that “that” indicates a previously stated idea and “this” suggests the idea and something new about it. Is that correct? How does that affect how they are being used in the above passage?

Clearly not interchangeable!

I see a very good difference in mentioning That is what and This is what.

The passage clearly mentions two instances. In first paragraph it explains about the law or procedure in place, which let us consider that won't be changed in near future. The other instance is an incident that happened in UK.

Now when we mention That, it refers to something little far away and This refers to something close by. In this case the law already prevailed before the UK incident which clearly means that the author wanted to differentiate the time difference of both the instances.

If That is what and This is what were used interchangeably, people might be think that UK incident happened first and the law was made later.

It is commonly understood that "this" is used to refer to something close at hand either spatially or temporally and "that" is used to refer to something further away. The distinction is particularly (only?) useful when a direct comparison is involved. For example, in comparing the Bolognese sauce one is eating now with a similar meal last week, one might say, "This is much richer tasting than that was." In the example cited in the question, there is little reason to prefer one demonstrative pronoun over the other. That said, the alternation between the two is awkward and perhaps even illogical, since both uses reference exactly the same situation described in the paragraph directly above. Maybe the writer wanted to avoid using the same word in successive sentences? An exact repetition would have been unobjectionable and even rhetorically emphatic irrespective of whether the choice was "this" or "that". In any case the repetition could have been avoided by rephrasing the second instance to replace the demonstrative pronoun with an appropriate personal pronoun: "When it happened to a family in 2013, it prompted..."