The DMG details the effects of adding a template to both a "normal" monster and an "elite" monster. No mention however is made of the effects on a "solo" monster.
By definition, a Solo monster is able to pose a challenge to an entire party of the appropriate level all by itself. Adding a template can only make it more able to do so.
But how much more difficult does it make the monster? Does it correlate to an increase in level? Or perhaps just a modifier to the amount of xp the encounter is worth.
We should keep in mind that the more we head up the ladder of monster difficulty, the more the rules are likely to become less steadfast. It is entirely possible that through practice we will find that the value of each template to solo monsters (or even among different solo monsters) may be very different. Frost Adept and Lich for instance offer very different benefits, and Lich will usually be the stronger choice, particularly when put on solo mobs.
But we will never know until we investigate.
There are several questions that arise however, about how the template would be applied in the first place.
Take Lich for example:
The Lich template adds
a) +2 AC/+4 Fort/+4Will - Fairly straightforward, these are simply added.
b) Some minor immunities and resistances - Again straightforward, add the immunities and resistances, if there is overlap, use the better of the two values.
c) Bonus to Saving Throws - Mentioned in the DMG, the bonus to Saving Throws changes whether you are adding the template to a normal or elite monster, so it stands to reason that it would be different when applied to a solo as well. But how? As a solo monster, +5 is already being added to saving throws, so the maximum benefit of this template would be another +5 (automatically succeeding on every saving throw).
d) +1 Action Point - Fairly straightforward, normal -> elite and elite -> solo give +1 action point, so this probably would too.
e) Hit Points - Another fuzzy area, as the DM guide says very different things for the normal -> elite and elite -> solo transitions.
f) Regeneration - Again straightforward, add the regeneration, if there is overlap, use the better of the two values.
g) Powers - Add them, if theres overlap, get additional uses.
So the big questions are Saving Throws and Hit points. So lets look at 3 points of view on the issue:
1) Minimum or no gain in these areas
2) Maximum gain in these areas
3) Average gain in these areas
1) Minimum - No saving throw bonus and no additional HP. Here we're adding a template, and we mostly want to just add on some cool powers and make the monster a harder to hit. The Monster is still pretty much just a solo monster.
2) Maximum - Monster automatically makes all saves and gets another 50% HP ( the amount it "gained" when it went from elite to solo). This makes the monster into a whole new tier of difficulty.
3) Average - Another +2 bonus to saving throws (+7 total), and another 25% HP. This is the compromise area, the monster is obviously a lot tougher than a solo monster, but not incredibly so.
Comparing those three options, I have to favor the minimalist approach. Both 2 & 3 trivialize saving throws, and with the boost to defenses, any increase in HP could stretch the fight to a point of boredom at best and TPK at worst.
So for my part in this thread, I will be assuming option 1. If anyone would like to argue that another option is more appropriate, I will certainly think over any suggestions.