So continuing on with this topic, I thought I would give an example of a really bad MM creature and a potential way of "updating it". In this case I chose something particularly challenging, the Purple Worm. It's just so relentlessly terrible in every manner, because unlike when updating something like a MM dragon you don't have a good powerbase or even great concept to make new powers on. Now this is the standard Purple Worms stat block:
As you can see it's absolutely terrible. It has one pitiful damage attack, a bite and it loses that for clasping jaws. Now the point of a solo monster is that it should threaten
five PCs. This creature is barely going to threaten one - let alone bloody one or even use its swallow. Bear in mind this is a creature designed around grabbing, but it has no way of holding its prey reliably because of all the conditions that can instantly end grabs: Daze, stunned and dominated for example.
But really, this creature will be making
2 attacks for the first two rounds (spending an action point to do so) and then it gets
one attack for however many rounds it takes for the PCs to inevitably kill it. It's attacks at the
incredible amount of damage of 2d8+7 damage. If is probably going to be lucky to bloody a single PC - let alone swallow anyone. Not to mention the high defenses the monster has as well, especially as this is a solider monster and back in the day, there weren't expertise feats giving you extra bonuses. So this is a
classic MM example of a creature with a lot of HP and high defenses making a grindy, boring encounter.
Bearing in mind the OP, how would I improve it? Well I would rebuild it from the ground up first.
To derive the above stat block I changed a few things, particularly thinking towards "How do I want this monster to function?". The purple worm seems like something that should be very good at grabbing a PC and then tearing them limb from limb. Unfortunately forced movement and stun - fairly common conditions for paragon parties to have makes that a difficult concept to reliably pull off. So the first two traits are both designed with this in mind:
Restless Earth: Inspired from the earthquake dragon in MM3, this aura prevents grabs being ended from the purple worm by PCs simply throwing it away with pushes, pulls and slides. It punishes them with damage and can help slide them towards the purple worm - keeping them within reach.
Ring Brain: Annelids in real life have a ring brain around their gullet and their nervous system can function very quickly - I figure that's a decent name for a power that generally protects the purple worms action economy. In the end is this reduces the severity of the stun effect on the worm - though it will end up dazed and slowed (which can in some cases be just as good).
This effectively is the "compensation". It will get reliable actions every round, but it relies on enemies being near it and PCs can exploit its lack of ranged attacks - without making it too easy (terrain helps, an underground cavern is my suggestion!).
Attack powers were next. When changing the original worm I moved Clasping Jaws to a triggered action that just deals more damage - giving it an additional benefit from having a creature grabbed. Technically this is a free attack - meaning the PCs may have to make a decision about triggering the aura to free an ally who has been damaged. As an added side effect, Clasping Jaws also automatically sustains the purple worms grab if required for whatever reason.
Tremorshock is a way for the worm to buy itself some room to move around in. Frequently solos can get "stuck" in a corner if they are huge sized, simply due to PCs flanking them and the solo finding itself running out of places to move. Combined with bite, it usually means it makes several attacks every single round if required - especially remembering that Ring Brain protects its action economy from being dazed/stunned. It should present a consistent threat - unlike the original worm.
Insatiable Jaws: This lets the purple worm turn anyone unfortunate enough to be nearby it into a one stop chew toy. Especially when he's bloodied.
Drag Under: This allows the purple worm to shift to a new position and then stun/damage an important target for a while. Often whoever is throwing stun and daze powers at it.
Swallow: I really hate swallow as every example takes a text book to explain how the condition will work and how it damages the PCs etc. It's absolutely iconic as an ability to the purple worm so I didn't have the heart to remove it.
Burrow and Burst: Gives it the ability to pop up in a new place when boodied, taking the fight to the PCs where they may have stuck it somewhere at a large distance while immobilized or similar. This lets it get out of any potential issues and then get into the PCs faces: Twice as angry and twice as deadly! Not to mention it will now have three insatiable jaws attacks - so it will have that extra damage bump while bloodied.
Summary of Changes:
I rebuilt the creature from the ground up instead of modifying it, but it lost a fair chunk of HP (780>624), defenses (My fortitude is 5 less! AC is similar though) and it has gained the ability to make much more attacks on its own turn (so considerably more damage). Damage of the primary melee attack went up from 2d8+7 to 3d8+11 (that's a pretty good hike). Additionally the purple worm I made might actually get to use swallow - of course a wizard with Winged Horde might ruin its day still.
Of course after some thought, I would already want to change the stat block I made as I'm not happy with it - but as an example I think it's reasonable. Especially when compared with "adjusted" MM2 creatures and other monsters. Many of whom have an area or burst power, usually a couple of triggered actions and some move/minor actions. So they can make use of many of their actions during a round.
I made this as a quick example, but I might make a second and better attempt at the Elder Purple Worm actually. The core point though is the differences in HP and in this particular case, power design that are required to bring a creature up to "par" with other monsters. Just a quick demonstration of the main points from the OP anyway.
Edit: On reflection, I picked the Purple Worm as an example of a solo because it's easy to show how all the new maths make a big difference. On the other hand, if I really wanted to redo this monster I would drop it as a solo entirely and make it an elite instead actually. It would function much better as an elite than a solo IMO.