They are well past their serviceable life and you would be re-tiling in a few short years.
Unless the tiles have been lying in storage for a few decades i think it would be impracticle to reuse them.
The idea was to match the existing tiles and then change the lot in 15 or so years (if they lasted that long). I might consider getting new tiles and doing the main house roof at the same time we put the extension up.
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