Main is main, dependent is dependent: these are 2 different visa types! You can switch from a main visa to a dependent visa inside the UK but can’t switch from ‘dependent’ to ‘main’.
One of the common mistakes migrants do, from our experience, is confusing the main visa with a dependent visa. For example, one spouse is on a Tier 2 (General) visa (that’s the main one) and the other spouse is on a Tier 2 Dependant visa (that’s a dependent visa). The visa ‘depends’ on someone else’s visa, so a Tier 2 Partner visa depends on the main Tier 2 visa. These are 2 totally separate categories: main is main, dependent is dependent. They can’t be combined when calculating qualifying period for Indefinite Leave to Remain (ILR, permanent residency) except for 10 years Long Residence which can be based on any type of visa. Most importantly, this matters to switching rules, ie applying to change your status inside the UK, rather than having to apply from outside the UK. You can switch from a main visa to a dependent visa inside the UK, such as from the main Tier 2 General to a Tier 2 Partner, from the main Tier 1 visa to a Tier 1 Partner. Actually, it would be the sam