Holders of Tier 1 visas in various categories and graduates with a valid Tier 4 visa can switch into Tier 2 visa in-country.
Recent graduates with a valid Tier 4 visa have the advantage of being exempt from the resident labour market test.
Holders of a Tier 2 visa who are changing employer need a new visa.
Also, if your job with the sponsor changes significantly, you may need to apply for a new visa to reflect this.