This seems like a continuation of that i270 Castrum Abania discussion from a couple weeks back.

But you know, I had a thought. Retainers! For standard Ventures, they require a light level for each level of Venture.
Currently, my Dragoon Retainer is only Lv.64, so I can't see the full scope of hunting ventures for up until 70. But I can see the 51-60 range.

At Lv.50, I had geared it with i130 Augmented Ironworks. (Leftovers from my own leveling).
Lv.51 - The Dusk Vigil (110 venture. Okay).
Lv.53 - Sohm Al (120 venture. Okay).
Lv.55 - The Aery (126 venture. Okay, but getting hairy).

At this stage, upgrading to Lv.56, HQ Crafted gear would sustain Leveling Dungeon progress. iLvl 139.

Lv.57 - The Vault (132 venture. Not okay).
Lv.59 - The Great Gubal Library (138 venture. Not okay).

I imagine when my Retainer hits Lv.70, I would be able to see a similar upgrade path for the Stormblood domain.

Which would mean the following. (Actually, found the rest of the retainer venture requirements from gamerscape)
Assuming a starting i270 Augmented Shire set.
Lv.61 - The Sirensong Sea (iLvl 245 venture)
Lv.63 - Shisui of the Violet Tides (iLvl 260 venture)
Lv.65 - Bardam's Mettle (iLvl 266 venture)
Lv.67 - Doma Castle (iLvl 272 venture)
Lv.69 - Castrum Abania (iLvl 280 venture)

At Lv.66, you can upgrade to HQ, Crafted iLvl 279 gear, which still wouldn't quite make the 280 cut for Castrum Abania. A couple pieces of gear from Doma Castle and Lv.68 HQ, Crafted gear would be necessary to bump you up to 280.
For the most part, I would say no gear upgrades are necessary for the 1st 5 levels of a new expansion (assuming capped gear from previous expansion). But you'll need to seriously look at doing a full replacement of gear after that.

Does this seem agreeable? To match up iLvl with Retainer requirements at level?