First of all, it is questionable why one needs a "change" to a job seeker visa if one already has a tourist visa. In a general case, it might be complicated to clearly separate tourist activities from job seeking activities. For example, this is exactly the reason, why US issues "combined" B-1/B-2 visas. So if one visits some sights, he/she is not prohibited to search for a job in parallel and sign a job contract.
However, a job permission with a proper visa or residence permit are needed on the first working day. If one is inside a country on a tourist visa, applying for residence permit should be generally not permitted. For example, this page of the Ministry of Migration and Refugees states that one needs a German national visa (NOT a Schengen visa) in order to apply for residence permit. Since in 99,7% of cases tourist visas are non-national visas, this implies that it will be almost impossible to apply for residence permit having a tourist visa. The one exception from this might be a Blue Card (I could also imagine some other exceptions like marriage or refugee status, but it is probably not relevant to this topic), which is issued according to European rules and therefore it might be allowed to apply for it while being inside the country. If one's job does not satisfy the Blue Card conditions, he/she needs to leave the country and apply for a visa in the embassy.
Please note that I'm not a lawyer and this is only my personal opinion on generic case.