All you have to make sure is if the motherboard will fit within the new unit and have a power supply that will support it. Using different computer chassis shouldn't be a problem as long as it fits within the unit and you have enough power for it. In the end, it's just another motherboard like all the rest mainly 

Not easy because the front panel is not single wires. 

Nor is the front USB ports standard 20 pin connection etc.

https://www.dell.com/support/article/us/en/04/sln293123/optiplex-990-mini-tower-mt-removal-guide-for-the-power-switch-cable-front-thermal-sensor-system-fan-io-panel-and-power-supply?lang=en

It could be done but would require some soldering and special cables.

  You will have to reverse engineer the part to find out exactly.  Removing the LED's allows you to visually see the polarity.

The front panel has 6 wires AND A Jumper?  Part Number KCRV8

You should be able to trace the wires from the assembly to get pinout.

Dell DOES NOT and WILL NOT publish this.

YMMV

The wire colors have NOTHING TO DO with voltages.POWER BUTTON WIRES POWER BUTTON WIRES

There are 2 wires for the on / off momentary button.

There are 3 wires going to a Tri color LED.

Careful disassembly of the cable makes this easy to figure out.

I am not however going to encourage this or spoon feed instructions on how to do this. Others have indicated that the red and yellow wires are shorted together through the switch.

Not having ALL wires correct will have "ALERT! FRONT I/O CABLE FAILURE"  Orange Black Blue would be the Tri Color LED.  Educated guess says that the black wire is the common connection for the CENTER of the tri color LED.

You will have to trace for the USB and Front panel Lights as well as the Audio cables.

Its not bad.  Its not easy.  But it is possible.

The only question I would have is why do you want a new case. Many have done it successfully.

Its not documented anywhere however and never will be.

Dell Fan wiring is the other issue.  They are not special just wired so that they self destruct when using standard units.

Its not standard but there are adapters.

https://www.amazon.com/Alloet-Connector-Adapter-Converter-Extension/dp/B07HF7YZ5W

RED is 12v

Black is Ground

Yellow/White is Tach

Blue is PWM

Dell does not use standard wiring or Pinouts on their fan headers for 3 or 4 wire fans.

For 3 wire they reverse 12v and tach. 

Black Red White not

Black White Red

DELL 3 PINDELL 3 PIN

This can lead to self destruction for a dell fan on standard motherboard or the other way round.

Sorry, my bad I thought they used the standard design like all cases but it's not the case from speedstep post. Pretty **bleep** stupid you ask me... I learn something new today.

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@905Peter wrote:

Sorry, my bad I thought they used the standard design like all cases but it's not the case from speedstep post. Pretty **bleep** stupid you ask me... I learn something new today.

 

Its been this way for years.  Another gotcha is the rear motherboard I/O panel.  It is not a removeable  insert on most Dell's, its built into the case.  The motherboard mounting holes are not all standard ATX, some will match up and some will not, although there are usually enough mounting holes to secure the motherboard. Dell also uses proprietary fans that are sensed by the BIOS.  If non-Dell fans are used there will be a "Fan Failure" detected on the POST (Power On Self Test) and the user will have to press F1 to continue with the boot.  No way to get around this with non-Dell fans.  

But, Dell isn't the only one.  I saw a recent post on another forum with an ACER that has non standard motherboard mounting holes, like Dell.  They were still waiting to hear from the user on the front panel connections.  However, I've worked (repaired) a couple HP's and those had standard front panel connections.

 

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Here is a pic of an Opti 990 SFF completely re-cased into a modern Tecware Nexus M mATX tempered glass case.  I brought everything over (temp sensor, case intrusion sensor, fans) except for the power supply.  a Corsair CX430 is providing power.  It was more of a mock-up project to see if it can be done and it surely can

MB is from a DT, not SFF.  SFF has fewer PCIe slots.

SFF MB:

I stand corrected -- DT it is