Hi
On the spec sheet pdf, here is what it says for the graphic cards:

On the setup guide, for the power supply:

I've both T3500 and T7500.  I'll open them later on and check the power supplies for each.  For sure they share the same slots type for the GPUs as I swapped between one and the other.

There's a HUGE PHYSICAL difference.

You cannot upgrade a 3500 to 7500

7500 power supply is not standard and the cables for the riser boards are not on a standard power supply so NO you cant do that either.

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_dell_precision_workstation/precision-t7500_service%20manual_en-us.pdf

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_dell_precision_workstation/precision-t3500_service%20manual_en-us.pdf

3500 on top 7500 on bottom.

Tiny 3500 VS HUGE 7500 

 T7500T3500 is much smaller

Hi

I assumed that you wanted a full new T7500 with all components in.  If not, quite few aren't compatible between the 2.

Just to share with you the power supply for the T7500 and T3500.

First the T7500:

Now the T3500:

I don't know if you can use a store bought power supply for the T7500.  Someone did it for a T3500 but I can't find any example for the T7500 either: https://www.youtube.com/watch?v=db7S1YmoJB8

The position of the screws to hold on the tower seem identical though I didn't measure the distance between each 4 positions.  Perhaps you can get an "official" dell power supply for T7500 from eBay?  There are few refurbished ones on sale.

I just went ahead and upgraded the video cards and power supply and installed usb 3.0 pci-e cards in 2 T3500s.  They are able to work with the mediasonic external drive enclosures i use. The T3500s support  Port Multiplier .   I did not want to take a chance on the 7500s not being able to support Port Multiplier .  There is quite a bit of info on the T3500s but very little on the T7500s. I also have 2 optiplex 960 towers as back up for anything that dies.The reason I want to use 3 rd party power supplies is getting stuff delivered here is problematic.  The power supplies and video cards and etc i can get via usps.  Which has done a pretty good job the last year.

But reguarding the 7500s if the power supply is dell only i would have a hard time getting a replacement.  Last time i checked trying to order a part from them they did not use usps.

The Dell Precision T7500 has all the base components to be converted in to a solid gaming computer.  Precision T7500’s are a single socket Xeon based workstations with an option to add a secondary processor on a riser board.  The T7500 supports up to 192GB of DDR3 memory and 4 Internal SAS/SATA Hard Drives.  It is important to note that it has a 1100W Power Supply, which gives you endless options for high end Graphics Cards. 

Recommended Graphics Cards:

EVGA NVIDIA GTX1660 Ti 6GB Graphics Card

EVGA NVIDIA GTX1070 8GB SC Graphics Card

EVGA NVIDIA GTX1070TI 8GB Graphics Card

EVGA NVIDIA GTX1080 SC 8GB Graphics Card

EVGA NVIDIA GTX1080 TI 11GB SC2 ICX Technology Graphics Card

EVGA NVIDIA RTX 2060 6GB Graphics Card

EVGA NVIDIA RTX 2070 8GB Graphics Card

EVGA NVIDIA RTX 2080 8GB Graphics Card

EVGA NVIDIA RTX 2080 Ti 11GB Graphics Card