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Having done this, it goes poorly. You create new problems as you go because somehow, to the surprise of everyone, brand new ideas are less fleshed out than vetted ideas. Shocking.

Making technical and functional changes results in an inability to explain A/B tests that attempt to prove that the migration did not have negative effects.

If you change UX as part of the migration, you're often unable to tell whether the cause was technical or functional. It's just too easy to say that it must be the technical side, but you don't have tangible data unless you actually compare the functional changes built on top of the same technical foundation.

Another aspect are behavioral metrics that may change between two implementations making these difficult to compare.

Agreed, you are almost always better off resisting that urge and taking note of things that should be improved as you go, otherwise scope balloons out of control.