Is it possible to upgrade the PCIe SSD?

I've asked OWC about PCIe SSD for iMac 21 late 2013, their answer:

I am sorry but we do not carry the correct PCIe SSD for the imac14,1. The only SSD option we offer is to replace your factory HDD with an SSD.

I replaced the PCI SSD on my late 2014 iMac 5K. It was the factory 128GB that I took out and replaced with a new 240GB OWC SSD along with a OWC 6G 2TB SATA SSD. The reason why I didn’t just get the 2TB PCI SSD is because the older models don’t have the higher PCI x4 interface. It didn’t make sense to get the 2TB AND then have to put something where the failing HD was. So I went with an ALL SSD “FUSION” setup which is possible with Apple APFS format. I am getting about 550 write/690 read and it is MUCH faster than before with the standard 3TB fusion. I also recommend putting new thermal paste on the GPU and CPU when tearing the system down. It was the first iMac I had ever done and it’s not at all difficult like they say. A little common sense and patience goes a long way. In fact, the most time consuming and tedious thing was removing old adhesive residue of the back of the 5K outer edges and on the iMac Aluminium itself! Get iFixit toolkit and it’s super simple with their guides! ANYONE can do it!