I toyed around with a similar idea, but in the end I figured it would probably be better just to give Summoner some extra quests for the Austerities of <Element> to get the new Egi, with something like "After mastering Dreadwyrm Trance, your aether has improved and now you can summon Ramuh-Egi!", they'd just need to add relevant offhand items to old quests (doing the Austerities of Fire quest, for example, would give you the item to equip to summon Ifrit-Egi, rather than Summon III being Ifrit-Egi. You'd equip it, and Summon, Summon II, and Summon III would all be Ifrit-Egi). Getting the "old" versions of those items into existing Summoners hands would be a bit of a problem, but we could have three new side Austerities quests (Lightning, Water, Ice) to provide the new Egis.
I have... other... ideas on how to boost old contents relevance... That Primal themed tomestone armor in 3.2 is likely just going to be "Grind some tomestones" to obtain, but I'd have much rather seen old Primal fights drop an item to grant access to new content; So you beat Titan, get an item, now you can face off a group of advanced, Lv60 Kobold Priests, who perhaps throw an advanced Titan-Egi at you, who has melee and mage actions, for example.
Of course, rather than the Austerities of Lightning being effectively the same as the previous three, it could involve a quest where we fight Ramuh a few extra times for Key Items to trade in to allow us to summon Ramuh-Egi, rather than some hand wave solution like I just suggested. We'd expand our aether to allow the rite to take place by beating the old guy several times, or something... Might even make working the existing Egi in easier, since we could have the Austerities of Fire (Hard) to unlock Ifrit-Egi glamour, with an offhand "focus" for the Egi being the solution to the aether problem we encountered when trying Ramuh-Egi (damn that quest was such a tease).