If I take the mythic path making my shield a legendary item, and choosing as one of the options "upgradable", do I have to choose which aspect of my shield to increase (defensive or offensive?) or does it apply to both the shield enhancements and the weapon enhancements I want to put on it?
EG:
I have done the above, My shield is currently a +1 spiked, bashing heavy shield with a +1 weapon bonus.
I pay 5000g (9000-4000) to upgrade it to a +2 spiked, bashing heavy shield.
Can I then, in a separate ceremony, upgrade it to having a +1 Flaming Weapon bonus (paying the 6000g of course)?
I would think so.
You're paying too much for your upgrades, though. The upgradeable ability lets you enchant it like your crafting. Increasing the shield enhancements from +2 to +3 would be 2500 (4.5k - 2k). Increasing the weapon part from +1 to +2 would be 3000 gold (4k -1k)