Car: 2010 Passat CC 1.8 TSI
Would it be a really big problem if I use the old cylinder head bolts or not?
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Your Passat uses torque to yield bolts, so they must be replaced. The torque to yield bolts are considered "one and done". This is because during the torquing process, they are stretched beyond deformation, past their plasticity. If you try to use them again, they will fail. If you reuse them, not only will they fail, but you will most likely cause damage to the head due to warpage.
Do it right and get new head bolts.
Replace the bolts, this has been covered in detail on this site : here is one such post with lots of detail :