Borg Vessel Assimilation
Borg vessel assimilation refers to the process in which assimilated vessels eventually develop themselves from the low-class Borg classes in-to the higher classes. All currently existing Borg vessels were once vessels belonging to other species which the Borg captured. They were then captured, assimilated and augmented with Borg technology in-to the first class of Borg vessel
Once an alien vessel has been disabled by a standard fully-equipped Borg ship, the alien vessel begins its own slow development to become a fully-fledged Borg starship by searching for vulnerable target ships and assimilating them. First the vessels engines are modified and enhanced along with the construction of a cutting beam, when the ship overwhelms another alien vessel its crew are beamed on-board the Borg ship, assimilated and further augmented in-to fully-equipped Borg drones. The vessels technology and materials are salvaged using the cutting beam and its components refined to be used again to enhance the Borg vessel in size, defense and firepower and to construct the additional technology required to support the new drones like alcoves. This cycle of search, assimilate and expand continues until the ship eventually develops in-to the first class of Borg vessel, a Probe.
Once the vessel has been enhanced and completely redesigned in to a Borg probe it is then assigned as a scout with secondary objectives to assimilate vessels of equal armament and firepower. The probe will then disable the vessel, assimilate the occupants and begin salvaging the materials which it will use to increase its own mass progressing in-to the next class of Borg ship. This pattern continues through the Scout sphere class, Tactical Sphere class and eventually expanding in-to the cube class vessel. The cube class whilst considered the pinacle of the Borg vessels evolution may also be further upgraded in-to a class-4 tactical cube class.
The Arctic Explorer
Original Arctic Explorer This shows the Arctic Explorer in its original configuration stated as being capable of a maximum of warp 1.4 and possessing no weapons.
Assimilated Arctic Explorer Phase 1 - 5 Hours Following Assimilation
Following several hours the Enterprise NX-01 encountered the Arctic One Explorer almost entirely covered in Borg armour enhancements and posessing newly constructed weapons systems capable of firing proton bursts and a cutting beam. Whilst having modified most of the primary systems such as engines, hull plating and having constructed new weapons nodes the aft hull plating and left wing remained unenhanced and unarmoured.
Maximum warp velocity increased from 1.4 to 3.9 within roughly 5 hours.
Assimilated Arctic Explorer Phase 2
In this phase the vessel is simply more complete, the left wing is now completed having added another weapon arm.
Assimilated Arctic Explorer Phase 3
The third phase the vessels maximum warp velocity was increased to 4.98 and overall mass is increased by 3%, bringing its appearance much closer to that of the Borg probe class. At this point pulse disruptors weapons were added and its defenses were further improved ("ENT - Regeneration").
"All currently existing Borg vessels were once vessels belonging to other species which the Borg captured. They were then captured, assimilated and augmented with Borg technology in-to the first class of Borg vessel"
The reason for the vessels ever-increasing mass was the vessel would have eventually developed in-to a Borg Sphere. (Star Trek Enterprise S2 DVD).
I adopted this idea that Borg ships always seek to grow and develop in to the next class because it seems inefficient and completely illogical for the Borg to use a class of vessel like the probe which is, small, weak and in no way superior to a larger classes. Therefore the idea of the small ones being the first step towards progressing to the larger classes is a more fitting theory. Plus it also begins to explain what class 4 actually meant for a tactical cube, assumably class 1, 2 and 3 were the stages of this enhancement cycle and class 4 was what we seen, the cube almost completely covered in ablative armour - which leads us to assume a class 5 would be a cube fully armoured with no visible openings anymore.