If I have a librem 14 that has the default coreboot + seabios bundle. If I purchase a librem key can i upgrade to firmware to the pureboot bundle setup? Or is it something you must select at time of purchase and no way to upgrade?
You can certainly switch over to Pureboot if you purchase a key. You can flop back and forth whenever you want 
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Is a guide or reference material that explains the process involved or how to do it?
Indeed, you can flash between coreboot and pureboot at will, however, only pureboot allows flashing from pureboot itself. If you need to go from coreboot/seabios to pureboot you have to boot a debian compatible distro and use the flash script as described in the docs.