I think they could easily add another tier of upgrades to the raid armor and weapons that would allow them to keep up with the new content. For instance the VoG armor would be able to get you to light 31 with an additional tier of defense upgrades. No need to buy armor if you already have a full set. The weapons should get the same treatment with an additional tier of damage upgrades. Then when the next expansion comes out both the VoG and Cota raid gear would get upgrades to light 33 giving the Guardian the option to choose what gear they want to use at all times. This should all be possible without having to reset progress and level up the gear from scratch, which isn't that fun as it is right now.

Armor starts at max light but less armor and or stats untill maxed to still give incentive to lvl them but not taking 4 steps back to go 5 steps forward