"Stock designs (generally those presented in the Technical Readouts and other CBT supplements) can be modified from their original specifications in two ways: via standardized "refit" kits or one-off custom modifications.
REFIT KITS
Refit kits are factory-produced packages, often created by the unit’s original manufacturer. The parts have been carefully selected to be compatible with a specific unit. Extensive supporting documentation is provided to make installation relatively straightforward. The disadvantage of refit kits is that they offer only a limited choice of options. Refit kits are graded by complexity. A relatively minor modification can be done in the field, but others are more involved and require a fully equipped maintenance facility or even access to the kind of support only found in a manufacturing center.
Standard refit kits are available for any variant of a specific design published in the Technical Readouts. OmniMech versions of older designs are always treated as a separate design in such cases. For example, a BJ-1 Blackjack can be refitted into a BJ-1DB, BJ-1DC, BJ-2 BJ-3 or BJ-4 model. It cannot be refitted into a BJ2-O OmniMech"
- Strategic Operations, p.188
"CUSTOMIZING
Customizing is the practice of installing non-factory replacement parts in a unit to improve or modify its performance.
Omni units are designed to use interchangeable modular pods, so they are rarely customized. However, even Omni chassis contain certain integral components, such as engines, armor and fixed weapons. These items are not installed in modular pods, so they must be replaced with customizing procedures. DropShips, JumpShips, WarShips and Space Stations can only be customized with the assistance of a functioning shipyard.
Some examples of custom refits are found on published record sheets. They can be identified by their designation, which includes the name of individual for whom they were customized; for example, "BLK-6-KNT Black Knight Ian" is the designation of a customized Black Knight.
(Warning! Performing customization of any unit will invalidate its warranty.)"
- Strategic Operations, p.189
Using factory replacement parts means the 'Mech stays stock - even if it changes variant. Using non-factory replacement parts means it's customised.
Simply swapping a Hellion Medium Laser (standard) for a Magna Medium Laser (+10 damage) does not make it a custom 'Mech (it still has a ML where the original specifications says it should have a ML). Swapping a Hellion Medium Laser for a Intek Large Laser does, if that particular chassis does not have a variant with a LL in place of a ML. If it does have such a variant, it is still a stock 'Mech.