I've recently started GMing a Rogue Trader game and I've come across a question about Weapon Upgrades.

If you have a gun with lets say 3 upgrades, could you acquire a new gun and swap upgrades (scope, silencer, etc) from your old gun over to the new one or would you have you accquire the upgrades at the time?

This is what the Armourer skill is used for. A failure might indicate that the accessory cannot be adapted to fit the existing gun.

Of course some upgrades like 'compact', are really derived for the weapon's design and so cannot be swapped. Upgrades like 'compact' need to be acquired as a 'combined' purchase - you cannot get a kit to miniaturise your bolter…

The rules don't spell this out (afaik), so it is up to the GM to use common sense. For instance, I find it difficult to imagine how some of the armour upgrades could reasonably be fitted to standard armour. Imo you would often need to purchase a new suit with the feature build-in. However, this leads to higher acquisition modifiers and so opinions differ.

Cheers Fresnel, that all makes perfect sense.

No Armourer in the group so this could cause tension.

I want to stress that you have an entire ship filled with personnell, though. Including a sizeable AdMech population.

You are also so rich that your fortune isn't measured in thrones, but in assets.

Even if you somehow are so inept that you cannot switch some very basic mods out (Scope, Silencer), you should be able to easily find someone - either on-ship or in port) to do it for you.

In my games, the seller can usually manage such things, as long as it is something that can be switched.

Logically the following upgrades are really easy to switch:
Scopes, red dots, underslung grenade launchers, melee attachments, suspensors, exterminator cartridges

The following will require some common sense:
Silencers only exchangable with similar weapons (a stub revolver won't be compatible with an autogun for example due to calibres),

And some just can't be exchanged:

Extra grip/pistol grip (is a modification of the weapons existing grips), compact (cutting down an existing weapon), vemon-drip (built into a melee weapon), lathe-wrought (a manufacturing technique).

Ultimately it comes down to common sense and not trying to powergame. If you want red-dots on all your guns, it doesn't mean you just swap the attachment in the middle of combat - you have to buy them for all your guns.