Changing from one visa category to another while in the UK is known as 'switching'. There are complicated rules around who is permitted to move from one category to another.
Tier 2 (General)/ Skilled Worker route
If a person is on a Tier 2 (General)/ Skilled Worker visa and wishes to change employers, they must obtain a new Certificate of Sponsorship from their new employer. The new employer must go through the same steps for employing any overseas (migrant) worker.
If a person is here on a visitor visa, they cannot switch to any other category while in the UK. The person must return to their country of origin and apply for a new visa from there.
If a person is on another type of visa, as an employer you should check the relevant Home Office policy guidance document for the visa they wish to switch to, to confirm it is possible from their current category.
Tier 2 (General)/Skilled Worker Route
There is no limit to the amount of time you can spend in the UK under the Skilled Worker Route (previously known as Tier 2 (General).
However for some routes, such as Temporary Worker or Global Business Mobility Route, an extension cannot be granted beyond a certain limit.
The requirements for an extension are similar to those for initial entry.
It may be possible to switch into alternative visa categories which might lead to permanent residence from Global Business Mobility Route.