Both of those questions, I would say, are a personal choice.. Yes, you could shift from veg to bloom nutes over a few days, or all at once, I don't think it will make a huge difference unless it's a super sensitive strain that requires a certain recipe. For instance my Forbidden Fruit strain, I've found, requires a slightly different recipe than most strains I've ever grown in the past. It likes a little extra K in veg and in flower, than usual.
As far as pruning, again it's up to you. Personally, I prefer to do any and all pruning right at the start of or just before flowering. Once my plants reach week 2 or 3 of flowering I don't prune or even remove any fan leaves.. Some folks like to remove lower branches or bud sites that will not get as much sun, so the plants can focus more on the main colas. Some folks don't. I reckon it's just a matter of personal opinion and what exactly you want to achieve out of your plants. If you're looking for as much biomass as possible and want more quantity of bud over quality, let it go. If you want bigger, tighter and prettier main colas, prune it up.