Update: I deleted my level 110 CdM, level 91 Zeus, level 84 CoA, and level 69 CC, along with my level 14 Star Gazer. Also placed into storage a Charcoal Express and 2 Olympic Treasuries. And sold 37 Sentinel Outposts, most level 2.
Replaced with various Inventory event/CS buildings, including a new Phantom Tower of Conjunction, new Festive Canal and new Sacred Archdruid Hut, all built from scratch. Also re-placed an Aegean Garden Resort, a Buccaneer's Bay, a level 9 Shinto Temple and a level 5 Sun Temple. And topped it off with upgrading a couple of Winter Wonderland Pyramids.
Boost changes:
Attacking army went from 1961/1464 to 1836/1289, a loss of 125/175.
Defending army went from 788/625 to 928/827, a gain of 140/202.
Overall FP production will undoubtedly go up, as will goods and other resources.
Bottom line here is that my loss of attacking boosts is at the very least offset by my increase of defending boosts, and the various other benefits from the replacement buildings will more than make up for the other benefits from the GBs I deleted. And going forward I can easily update each building as I enter each new age/era. There's really no need for the Star Gazer, which only gives previous era goods, as I have plenty of Stages of Ages, both in my city and in Inventory, which give previous era goods and other benefits/boosts.