Switching from Chancenkarte to Employment Permit

I have a question in regards to the process of switching residence permit within Germany. I am a UK citizen, currently living in Germany under the Chancenkarte (specifically with a recognised university degree if that matters!). I got my visa from the UK embassy in June, moved here, and then got my residence permit from the Essen immigration office.

My question is, what exactly is the process when it comes to switching to full time employment? If I was offered a full time job, do I need to reapply for the correct visa (if so, can I do this here or does it have to be back in England), or can I just apply for a change of residence permit at the same Immigration Office? It's very hard to get a straight answer online, and even harder to get advice from the office!

Also, if I have a German job offer, do I still need to prove B1 German language level? Will this be a part of the above new visa/residence permit application, or do they not care about language level if you have a job offer?

Finally (sorry!), does the job have to be related to my degree? Or can it be any field?

Vielen Dank!