I don't know how you were left with that impression, but that has NEVER been a supported option. Not for your Mac or any other models.
Drives could be moved into new Bays in a newer model (if so equipped). PCIe cards could also be moved, but in the years since the original 2006 model, the original drives and original PCIe cards are completely obsolete.
Hobbyists have force-downloaded the firmware from the Mac Pro 5,1 [RISKY!] the 2009 model and if it does not get "bricked", it keeps working as a 5,1 -- slightly extending its life.
After the firmware upgrade, if you also upgraded to a Metal-capable graphics card, you could consider running 10.14 Mojave.
Along with that, some hobbyists have upgraded the processors on the firmware-upgraded 5,1 model. But if you have the DUAL-chip model, only Lid-less processors will work, and you would have find some de-lidded or do it yourself. Requires many razor blades and risks cutting your fingers badly!