EVE New Citizens Q&A

It's only really rarely done because either a) the fight has already started and you'll lose damage by switching types or b) People don't memorize resistances well enough to really take advantage of them.

with proper knowledge of the target ship it's certainly possible to hit the weakest one. "PVP ships are omnitanked" does not mean "All of their resistances are the same, or even close". Good luck getting a Wolf with kin/exp resistance anywhere close to its thermal/EM resistance, for example. In fact, using EM against a Wolf is suicidal (90% base on armor).