Can visit visa be transfered to work visa without leaving UAE ??

No it's impossible to do so. Against the law.

It totally depends.best to speak with the PRO of whoever is issuing your visa. I have never had to leave to exchange from visit to resident or employment visa, however my husband did, and it was not as easy as driving over to Oman and back. He had to fly to doha and back as you must enter through an airport (also do not for example, get a cheaper flight thru AD if you visa is a dubai visa, fly into dubai airport... one of my employees didnt do this and this caused us months of problems) Depending on your nationality you may need to be away for a certain period. For example a south African I was arranging a visa for ended up being away for a few weeks (due to public holidays etc).

The immigration department are actually very helpful if you go down to the offices and speak to them...do not bother going on the phone tho. You should however, be able to get all the info from the company you go to work for, or if you are setting up alone, the business registration company.