If the pets are that close without pet switching I have a hard thinking how hardcasting a pet and using a soulshard to summon a new one immediately would give you any type of gain, especially considering the only thing that makes pet swapping viable for demo the fact that felguard has felstorm and an amazing demonsoul to make up for the lost time spent casting, and the imp and felhunter as of now with shadowflame are practically identical to each other overall. The closest thing I can see to pet swapping in live is maybe starting with the imp for a better demonsoul at the start and then using a single soulshard to instant summon a felhunter and then use the felhunter for the remainder of the fight, and even then I have no idea if the GCD you spent summoning the felhunter would even be offset by the damage gain you get from having an imp DS instead of felhunter DS at the beginning of the fight. It would be an insanely marginal gain and require you to be in melee range for the entirety of the fight if it were to provide any type of gain at all.
The reason imp is so much better with 4.3 is because of the massive buff we're getting to burning embers and the increased ISF duration, meaning that we'll be casting soul fire less and as a result we'll never see our burning embers hit its damage cap from soulfire alone to take advantage of the buff in the same way we will be able to with an imp keeping it up 100% of the time.