Originally posted by @guitargeek in #7859 (comment)

Hi @hageboeck! For the schema evolution from the previous version, I just made a test myself where I wrote a RooDataHist to a file with ROOT 6.24 and read it back with ROOT master + this PR. Do you think I should implement a unit test for that? I didn't think this was necessary, after all the change is rather trivial (removal of _realVars class member).

I see that I never really replied to the above:
Yes, I think that kind of a test like this makes sense. Consider this:

• stressRooFit has a reference file that reads in some RooFit objects. This kind of serves as a schema evolution test, but that's more by accident than by design.
• The reference file evolves with ROOT, though! When you find that small things have to be changed, you recreate the reference file by running ./stressRooFit with the correct arguments.
• Now you are not testing 6.2[02] schema evolution, but whatever was in master when you created that reference. This is not a good and stable test.
• Here's a test suite that tests the I/O of categories across several ROOT versions. It's trivial to extend it by creating another reference file and adding it to the test suite:
  

  root/roofit/roofitcore/test/testProxiesAndCategories.cxx

  Lines 57 to 130 in 2ef9c05

  	class RooCategoryIO : public testing::TestWithParam<const char*> {
  	public:
  	/* Test the reading of a simple mock category that has the states
  	* one = 0
  	* two = 1
  	* three = 2
  	* four = 3
  	* The ranges "evens" and "odds" for even and odd state names are defined.
  	* Now, we check that set ranges are read and written properly, and that
  	* sharing of those ranges works even after reading back.
  	* A mock file can be created as follows:
  	RooCategory cat("cat", "a category")
  	cat.defineType("one")
  	cat.defineType("two")
  	cat.defineType("three")
  	cat.defineType("four")
  	cat.addToRange("evens", "two,four")
  	cat.addToRange("odds", "one,three")
  	RooDataSet data("data", "a dataset with a category", RooArgSet(cat))
  	data.fill()
  	TFile outfile("/tmp/testCategories.root", "RECREATE")
  	outfile.WriteObject(&cat, "catOrig")
  	outfile.WriteObject(&data, "data")
  	*/
  	void SetUp() override {
  	TFile file(GetParam(), "READ");
  	ASSERT_TRUE(file.IsOpen());
  	file.GetObject("catOrig", cat);
  	ASSERT_NE(cat, nullptr);
  	file.GetObject("data", data);
  	ASSERT_NE(data, nullptr);
  	catFromDataset = dynamic_cast<RooCategory*>(data->get(0)->find("cat"));
  	ASSERT_NE(catFromDataset, nullptr);
  	}
  	void TearDown() override {
  	delete cat;
  	delete data;
  	}
  	protected:
  	enum State_t {one = 0, two = 1, three = 2, four = 3};
  	RooCategory* cat{nullptr};
  	RooDataSet* data{nullptr};
  	RooCategory* catFromDataset{nullptr};
  	};
  	TEST_P(RooCategoryIO, ReadWithRanges) {
  	for (RooCategory* theCat : {cat, catFromDataset}) {
  	ASSERT_TRUE(theCat->hasRange("odds"));
  	ASSERT_TRUE(theCat->hasRange("evens"));
  	EXPECT_TRUE(theCat->isStateInRange("odds", one));
  	EXPECT_TRUE(theCat->isStateInRange("odds", "three"));
  	EXPECT_TRUE(theCat->isStateInRange("evens", two));
  	EXPECT_TRUE(theCat->isStateInRange("evens", "four"));
  	EXPECT_FALSE(theCat->isStateInRange("odds", "two"));
  	EXPECT_FALSE(theCat->isStateInRange("odds", four));
  	EXPECT_FALSE(theCat->isStateInRange("evens", "one"));
  	EXPECT_FALSE(theCat->isStateInRange("evens", three));
  	}
  	}
  	TEST_P(RooCategoryIO, TestThatRangesAreShared) {
  	cat->addToRange("evens", "three");
  	EXPECT_TRUE(catFromDataset->isStateInRange("evens", "three"));
  	}
  	INSTANTIATE_TEST_SUITE_P(IO_SchemaEvol, RooCategoryIO,
  	testing::Values("categories_v620.root", "categories_v621.root", "categories_v622.root"));

So the answer is:
It would be preferable to have a similar test to what I pasted above for RooDataHist. A stub is already here for RooDataHistv4:

By making it a value-parametrised test as for the categories, you could instantiate it very quickly for all RooDataHist versions.