Cases are iffy...because they come in different sizes.
You cannot put a motherboard designed for a larger case...in a smaller case (safely).
Power supplies can also be iffy, but generally...they can continue to be used. The wattage of the PSU is important, if you intend to game or are adding components which will draw additionally on the PSU...but if you are merely moving/replacing the guts of the system...the PSU should be a non-consideration, IMO.
What I usually do is buy a new PSU when I buy a new motherboard/CPU...and I keep the old one for an emergency backup.
I also buy a spare CMOS battery, just in case the new one turns out to be an "old" one
All drives and any add-in cards can also be used...if the new motherboard has slots for such (connectors for the drives). We now have two types of drives (PATA and SATA) and two types of connectors. Newer boards may not include PATA connectors or may be limited to only 1 (which will support a total of 2 PATA devices).
All in all...it takes a little thought by the user. It's not like changing a spare tire because it's flat.