And yes you can gradually introduce fitch and have a mix of the two
I did that with our last hamster until eventually it was all Fitch. He did prefer the Fitch. It's nice and soft and they add it to their nest along with regular toilet paper strip nesting material.
As Cypher says, wood shavings in the Uk have to be kiln dried by law, which reduces the amount of phenols in it. It should also say "dust extracted" on the bag. I always found them a bit dusty and was much happier using Fitch (which works out cheap to use as well).
But there is no risk to your hamster in the short term. Using the wood shavings permanently, long term, is something many people are not agreed on.