Like an epitaph to be unchangeable (6,2,5)

I believe the answer is:

carved in stone

I'm a little stuck... Click here to teach me more about this clue!

'like an epitaph' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'be unchangeable' is the wordplay.
'be' becomes 'carved' (I can't justify this - if you can you should believe this answer much more).
'unchangeable' becomes 'instone' (I've seen this in another clue).
'carved'+'instone'='CARVED IN STONE'

'to' acts as a link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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