Can I replace a Quality Enchantment?

Yes, you can.

Rules regarding enchanting, from what I've gathered are as follows:

• you can have only one enchantment per category
• enchantments are restricted by the anvil symbols - each piece of equipment can hold a maximum of 12 anvils.
• enchantment not available for custom enchanting (and therefore not listed as taking a slot under any category), unique ones that some weapons you can aquire already have, do not influence it (except for the rule above) - they will not be lost whatever you do.
• enchantments are not exchangable - once you get one that's it, slot of that category forever stuck in the choice you made. Quality category however is an exception to that rule:
  • You can freely exchange enchantments in Quality category provided you change it to an enchantment that is equally powerfull or more so (the new one has to have greater or equal amount of anvils than the one already there). For example Fine can be exchanged into Accuracy 2 (and the other way around ad infinium), but Exceptional can only be changed to Superb