Can I update my internet speed without loosing my earlier subscriber advantages?

Unfortunately, no. Once you change your speed your change your contract. Wish I had better news. However, try to reach out to support, maybe they can do something for you on a loyalty basis.

but he will use his special price that he got for signing up early. That's what his question was about, wasn't it?

Oh is he talking about the bonuses your normally get for being a fizz subscriber? These you keep but not your special pricing that you got as an early subscriber, when Fizz started and was in the testing phase.