Are you actually asking?: "How hard is it to switch from sole proprietorship to s-corp/llc if you keep your accounting straight and use your personal bank account?"

Not hard at all. If you are a sole prop, American citizen, paperwork in order then it is very easy and a law firm could do it in a couple of k. Orrick has Start-up docs on their website for free.

However, remember that going from an LLC to anything else is very expensive. This varies state to state, in the USA. We formed an LLC in Virginia. Our lawyer warned us that if we ever wanted to go to Wall Street and become public, then a standard C form corp was far more appropriate. And you can not easily transition from LLC to C form corp. Our lawyer told us that the IRS would treat is as liquidation sale and tax everything we had - in other words, it would never make rational sense to do this. It would be as if our LLC went broke and was liquidated and then was bought by the new C form corp.

When we closed down that LLC, there was intellectual property we had to entangle, so that it could go back to belonging to various individuals, all of whom were continuing with their careers. This was messy. We had a highly customized partnership agreement for our LLC. That is the great thing about LLCs - that they allow such flexibility (this, too, varies state by state in the USA. Virginia allows a great deal of flexibility). My advice is, if you form an LLC with other people, put in a clause about shutting down. Essentially, negotiate a pre-nup. Divorce goes easier when everyone knows ahead of time exactly how it will be conducted.