As a returning player who has made a couple froob chars, I have naturally been epxloring the various possible froob-friendly weapons. Among other things, I have noticed that some SMGs have been chaned since I last played. For example, OT Kerans became “Kerans Automatic Grinner - 404”, and instead of the old Mausser Particle Streamer, the shops carry “Mausser Particle Streamer - 404”. There are also several other weapons with numbers added to their names. The new versions seem to have better stats than the old ones, which is great. My question now is about the following line that is common for all those new upgraded weapons: “This weapon can be upgraded further. It is possible to balance the weapon so it fits you perfectly.” And, indeed, when I put either Kerans 404 or Mausser 404 in the tradeskill window as a target, the descrption says “Used in 1 different processes”. So, does anyone know how to upgrade them further, and what items are used as source items in those processes?
And, before anyone mentions the old procedure for making chemical streamer from a particle streamer, it is not that. I tried to hack a Mausser 404 with a hacker tool, like it was possible with the old version of mausser particle streamers, and it no longer works.
Firstly let me welcome you back to AO, Valvus.
Secondly those weapons in shops are upgraded RK weapons. Their base are still the good old RK weapons, such as OT Kerans Automatic Grinner as you mentioned. These old-school RK weapons can still be obtained from mobs all around RK and in RK missions or they can be rolled as a mission reward.You can then upgrade such weapon further using Weapon Upgrade and Weapon Adjustement Kits.
For the full guide please visit: https://www.ao-universe.com/index.php?id=14&site=AO-Universe%2FKnowledge%2F&pid=354&lang=en
Hey hey 
For ranged weapons you need:
And for melee weapons:
Note that these are new weapons and not replacements. The original Mausser (and any of the other weapons with a 000 suffix) is still available in weapon shops (as opposed to weapon terminals in general shops) and rollable as mission reward. It can also be built from scratch. Have a look here for the recipe:
Ah, great, thanks, that is exactly what I wanted to know.
Update: actually found this item ( Ranged Weapons Adjustment Kit) and tried it out with one of my weapons. The requirements for burst/smg stay exactly the same, while DPS does go up a few points.
Update 2: in spite of what the description may lead one to believe in, the resulting weapon does not become nodrop.
They used to become nodrop, but it was changed and the description was forgotten 
How ever the concerning part with the new weapons is the early lvls and the ridiculously long cool down for specials. Due to that the Basic shops should have kept the RK weapons untouched.
The Trader shop weapons are just gone, not moved to any shop in ICC like the Fixer shop stuff.
There is base RK pistols with burst some times on it’s own or with fling, and some times with SMG added. But all the new ones are just fling with nothing else.
All the new AR load outs are burst and FA, no fling on any or burst only or burst and fling.
The original weapons are sold by the vendor NPC’s in the specialised weapon shops, for example next to antique shop in Borealis. They’re also rollable.
Are you referring to any specific one? For example the Russian Good Day, Tango Dirk and BBI Faithful are rollable. I don’t think there’s anything that is no longer obtainable (though it’s not as convenient).