I currently hold a UK short term tourist visa. Recently we have decided to marry with my partner (who lives in UK), who I've been together with for 5 years. For us to register the marriage in UK I need a Marriage Visitor visa, also known as fiancé visa. My tourist visa expires in March, but we'd like to get married earlier, in January.

Is it possible to switch from tourist visa to the fiancé one before the tourist one expires?