It doesn't change from person to person. The baseline in the unknown would be that there is no evidence of its existance. If you are deviating from that and saying without a doubt it does not exist, you are deviating from the baseline and must prove your case. If some says it does exist, they are deviating from the baseline and must prove existance. It would be no different that me saying without a doubt there are no other planets in existance other than what we have discovered. The burden of proof is on me then to prove that bold statement. If I were to say there are definitely other planets we have not discovered I would need to prove my case. Both are deviating from the agnostic baseline of no proof exists on either side