Well I found the part of Battletech books I was looking for.
I now know MatthewPryde needs to read more rule books for Battletech
They make mech mod charts to allow you to retrofit battlemechs (as in, upgrade a Warhammer 5DSK to a 7DSK) and swap out omni ports on omnimechs. Like change a mad dog's arms from lasers to missiles. They are not the swap and trade Armored core esque things found in the mechwarrior games of late. Light mechs are great, faster mechs are excellent hit and run mechs, as you can see in MWLL they can be the scourge of larger, more focused units, and are the best at handling an attack from personnel units like BA, but the LAMS does not accomplish any of this. It was simply an early addition when they were stealing from Macross, and phased out asap.
No! To refit from from a Warhammer 5DSK to a 7DSK you buy the refit kit

(From Mercenary's Handbook 3055 (1670))
Now this rule is for starting Mercenary group, but as you can see, customization is part of the Battletech Game.

(From Battletech Field Manual (1701))