A lot will depend on which system you're tracking this with, I think. The good news is that for what you're asking, it seems to all fit quite tightly in to place in a 1:3 replacement in the Side Torsos. This will allow you to effectively either increase your rocket firepower by 50% or your dwell time.
If we go by Campaign Operations, this will be a Class B Refit, maybe? Since you're removing stuff from both Side Torsos and replacing them, this seems to fit. That means it can be done in the field, (though, you may still want to get your best facility if you can), and doesn't automatically reduce the Quality of the unit (though, the Roll itself might do that).
This is my first time running these numbers, so bear with me.
It will be a Technician Skill Check.
A Class B Refit provides for a +3 modifier (pg 211). It also triples the time it would take for this refit. It's not from a refit kit, so no other modifier from that.
It will also include modifiers from the Maintenance, Repair, and Salvage Check Modifiers Table (pg 192-194). Rocket Launchers are Tech Rating B, so apply a -2 Modifier.
There's a Unit Quality Rating Modifier. In this case, the more "New from the factory" fresh it is, the better the modifier. This can range from -2 for a pristine Rating of F, to a +3 Rating for A ("it's been salvaged how many times now?").
There's also modifiers for when you are, and which faction you are. In most cases, outside the Jihad, you're looking at a +1 Modifier, except for the Late Succession Wars (3rd and 4th) which are +2.
You can put up to 3 teams on a unit for a -1 for each additional Team (using teh best team's experience).
If the main tech team has personnel issues (casualties or hits), this can in crease the modifier as well, up to a maximum of +4.
Location will matter. An Improvised field station will be a +2, but if you're in your Dropship bay, that is a null modifier of zero.
Then there's planetary conditions, but most of that is crazy weather, times, atmosphere, or even in the middle of nowhere (i.e. zero-G). You didn't mention them, and being in a Dropship helps ignore most of that.
Depending on when you are doing it, if Rocket Launchers are just back in vogue, then you might need to add a +2 modifier for "Extinct Component". Since you're using a 'Mech that can already use them and isn't a prototype model for that, I think we can ignore that modifier.
From there it will be based on how much time you give your tech crews to complete the job. Doing overtime, jury-rigging, rushing the job, or even just taking extra time to do it will affect the results (hint, taking your time is better).
So, assuming a Merc unit in the Jihad, with a Quality D unit, doing a normal job, we have:
Tech Skill Base
Class B: +3
Tech Rating B: -2
Quality D: +0
Era Modifier: +0
Full Team: +0
2 more Teams: -2
Basic Facility: +0
For a total Roll of Tech Skill - 1. Not too bad. However, there were a lot of assumptions being made there, both in system and certain qualities being involved.
