I'd fathom that HOs/sHOs/DSO/etc could be added to a new IO category that is comprised solely of HOs and their like to enable them to be convertible via converters.
Otherwise an NPC to trade them for others. Maybe they trade them in for a token. Which is used to buy them from the vendor at a tier range going from 1:1, 2:1, 3:1, 4:1, 5:1.
What balance reasons are you referring to? Making HOs of all types more accessible for everyone seems like an equitable ideal that seems in line with the very nature of balance.
But then again maybe you're referring to the existing economic stratification of the player base that makes it more difficult for newer players to be able to slot things like veteran players?
Regarding your suggestion about stockpiling...Yes I have a stockpile of enhancements, as I've been here for a long time, which I assume is obvious given my 3k+ post count, have close to 300 lvl 50s, and the majority of all my toons are IOd to the gills.
But hoarding the wealth doesn't reduce the economic barriers that newer players face in being able to build our their characters. So for that reason having the ability to convert a HO to a more useful type is balanced (as the effort required to get them is the same whether it's a Centriole or a Cyto, or a Amylophast vs an Ectosome).