If it's direct transaction from altcoin to bitcoin with no any fiat in the middle or any bank transaction, you won't need to pay for taxes because they can't track that.
It should not be that simple. As far as I know, if there is earnings, the tax have to be paid. The rules should be thoroughly researched on google, then should make decision.
maybe for now only a few countries are applying to pay taxes or maybe there is no such country.
but from what I hear, crypto owners will be taxed every year such as vehicles and homes.
I have not confirmed whether it is banar or not. maybe someone will explain later to you.