The only problem I've encountered in the past when swapping out a power supply, was that I could not physically get it out of the case without removing the CPU heatsink. Some cases, I'm thinking of those with top-mounted PSUs, are designed with a rail to support the PSU such that you have to turn it sideways and move it down across the CPU in order to get it out. This usually requires removing the CPU heatsink. To me that's a lot of extra work. That's another reason I like cases with bottom-mounted PSUs, in addition to the case being more stable (not top-heavy and prone to tipping over).