The Achilles is exactly what I had in mind, but it applies to all the 'assault' dropships from 3025 and 3057 like the Avenger, Kuan Ti, and Claymore. They might as well be retired in the 3145 era if they can't be drastically redesigned. Of the lot only an Achilles could survive a hit by a Taihou's heavy SSCs.. barely. And even then only if the shot hit the nose armor.
You might argue that the pre-3145 'assault' dropships are relegated to solely a fighter-killer role now rather than still being true assault ships. But a squadron of heavy and/or XL fighters will make short work of their 200ish armor facings just as fast as a PWS, and are probably faced more often. Particularly if the dropships are meant to hunt fighters.
How WOULD one retrofit them to make them relevant?
Using the example above.
Well... I would start by noting the Taihou is a 12,000 ton DropShip and the Achilles is 4,500 tons. You wouldn't expect a Jenner to take on an Atlas alone?
After that you need to start thinking about role. As noted above sub caps don't really change things here. The Kuan Ti and Merlin variants show this where their sub cap variants aren't notably more effective than their traditional forms. The big change is that nations are now building 8,000, 12,000, or even 36,000 ton DropShips just for the combat role. A 36,000 ton DropShip is going to scary whether it mounts SCCs or PPCs.
The other thing is that XL engines don't really change the ASF:DropShip equation. It sucks to be the DropShip. In any time period your job is to absorb damage, keep evading, provide some ECM, and hope your fighters save you.
So basically smaller assault DropShips are back to swarming tactics. Hit the big ships with numbers and hope you survive. Upgrading armour would be nice, but often size prevents it. Remember the Nagasawa? It badly needs some more guns, but goodness knows where they would fit.
The Nagasawa (and Interdictor, and Isegrim) really show the future of the light DropShip. The Nagasawa is the spiritual successor of the Noruff as the Interdictor is the descendent of the Achilles. Pack on the pounds to carry the armour needed to survive. Yet for all of that the older, smaller craft are often just as effective on a power/tonnage basis. They just have trouble clearing some damage thresholds in the same way Clan PPCs ruin light 'Mechs worse than medium 'Mechs.
Food for thought.