@Nami, The best way to find out is to email Purism and ask if you can change your order to Fir. Then, let us know what is the response.
As of Nov. 2020, Purism was still saying that Fir would be coming. NXP just started producing the i.MX 8M Plus a couple months ago, so it came about 2 years later than expected, so I’m not surprised that Purism hasn’t said anything publicly. Considering how long it has taken for the i.MX 8M Quad to get mainline Linux support and how many new elements are being added in the Plus (neural processing unit, image signal processor, digital signal processor, advanced 3D audio & hardware video encoding), it will likely take a while to support these new components in the mainline Linux driver, so I do wonder if Fir will even use the Plus.
Instead of upgrading to the Plus, my hope is that Fir will use the existing Quad processor, but add more RAM and Flash storage, and upgrade the USB 2.0 bus to the microSD card to USB 3.0, and add hardened glass to the display, plus reduce the weight and thickness.