Renewing working visas

If you already have a current working visa, can you renew it while still in Poland? Or do you have to go back to your home country and do the entire process again?

You can renew it and it has to be done by your employer as far as I know. It costs some money to file your request.

The work permit is fairly easy to renew (or at least easy compared to getting it in the first place) but your employer needs to do it. However, can not renew your visa with working rights (i.e the sticker in your passport which you got a the Polish consulate in your home country). You must instead get a temporary residency permit and those can only be got in Poland. Go to your viovoship foreigners office and apply there but do so at least 45 days before your current visa expires.