Hello quick question I already own the original One unit whole blood release on here is there a way to upgrade my version or am i stuck with only buying the new release? the store page for one unit is gone now it seems.

If you have the original One Unit Whole Blood here on GOG, then you get a 5 USD discount on the Fresh Supply remaster. You'll then have both within your GOG shelf, though the One Unit Whole Blood becomes obsolete as this is part of the goodies for the remaster.

How do you get the discount?

It's applied automatically if you own the OUWH on GOG.

Non-original owners get a regular 50% Discount as well.