Will old cases fit new parts?

MADVIU

Distinguished
Yes, there are a number of ways that you can tell if the parts will fit or not.

The easiest but possibly most troublesome would be attempting to install the new components in the old case and see what happens. If they happen to fit, then you are good, however, if they do not, you now know that they do not fit and you will need to explore another avenue.

The smarter way to go about this is to obtain the dimensions of the case that you have (knowing the exact model is helpful) and then relaying that information to the parts that you plan to install within the case and then making a judgement call as to whether or not they will fit based on the dimensions. There are some cases that you can remove a few bays that may make room for longer GPUs, but beyond that, if you have some things that are simply too large, then you are pretty much out of luck.

Listing the exact components and case dimensions would be most beneficial.
 

punkncat

Polypheme
Ambassador
The "two" important things, and a few lesser.

First, check to see what motherboard format your case was "designed" for. Most of your ATX mid + sized towers will hold and ATX or micro ATX mobo. Below that tends to get a bit more involved. In some cases where a tower was designed to hold multiple formats, the screw holes for the mobo will be marked as to that format. You mention having an aftermarket case, which is generally good for using a standard format. Be aware that some of the 'brand name' cases use screwball mobo formats.

Next, check the dimensions of your graphics card, cooler, and verify what PSU it's going to use. Generally you want to know the length of the graphics card. In the case of most ATX/Micro mobo it's height won't matter. Know the height of your cooler in reference to how it sits in the case. Is your PSU going to be in the way, etc.

Lastly, consider what drives you will be using and where/how they go in the old case. If swapping from 3.5 to 2.5 do you have the places to put them? Need adapters, etc. Its also a good idea to verify that the plugs on your PSU are what is needed by the newer mobo, GPU, and if really old your drives.
 
Unless it is an ancient BTX design, which is unlikely, then count the number of expansion slots it has in the back.
A ATX case will have 7 or perhaps 8. A M-ATX case will have 4.
If your motherboard has the same or less expansion slots, , you are good.

There are unusual situations where a very long psu(say 180mm) may not fit, or a very long 11.5" graphics card will not fit.

It is probably not worth doing anything prematurely and buying a new case.