I would recommend that you keep at least one Elite War Hero Relic of Boundless Ages. It will still give you consistently more power than the Arkanian relic of Boundless Ages. Noticed the underlined word. Unless you are constantly hitting your target with the best you can offer every time you proc the pve relic of boundless ages they will perform about the same. Not to bore anyonebut, in a constant fight and the ideal circumstances where you can hit the Underworld relic of Boundless Ages as soon as cool down refreshes it will only give you around 1 to 6(?) power per second more than the old Elite war hero ones. This can come down to preference if you prefer to see your numbers to jump and expect that every time a boundless ages procs that you will be doing wonderful burns then the new PvE one will be better.
On the respect of the second one, it will be beneficial to replace it with a Serendipitous Assault relic as these have a very nice increase in terms of power per second over both the Elite War Hero and the Pve Boundless Ages.
In the skimpy version, keep at least one to experiment with against the New boundless Pve to find your preference, and replace the other with a Serendipitous Assault.