As usual, mutiple choice.
Here are the choices:
Choice A - Upgrading RE4 style, gravely-voiced british merchant
Choice B - Upgrading same as Choice A, however weapon statistics are NOT shown on inventory screen. (Alternate New RE)
Choice C - Upgrading RE2/RE0 style, finding parts to combine with your weapon. Statistics will not be shown. (Old-School RE)
Choice D - Upgrading with parts, however parts are bought. Statistics will be shown on inventory screen. (My own idea)
Choice E - Other (Say in thread)