I really don't like it- I believe that single target long range AA should go into cyclone and make it cheaper/more disposable instead of the current mess of the lock on mechanics.
Even if the above wasn't possible, Thor being single target could use more help in that department as it is losing the splash damage- Numbers really don't make up for the loss of splash
Now onto my points:
Thor change:
Thor change is really overall nerf to utility of the unit- Thor is 6 supply unit that does 35+15- numbers may look impressive at glance, but it really is population inefficient. Its not really useful at all but vs broodlords- which it doesn't do great against since both sides can kite each other and broodlings block the thors. Thors also have clunky firing delay that worked with burst damage nature of the javline missles, but doesn't work as well with faster-firing current change missles.
Not only that, the damage isn't really great- 25 air DPS vs armored for 6 pop unit isn't really "worth" it when its huge, blocky (blocks other thor from firing at same time) when vikings can stack and do just as well and offer fast, reliable single target focus-
You may argue that Parasitic bomb is a big threat for the vikings, but binding cloud does just as much vs AA thors in denying damage. There is no real reason to make thors against lategame air except in TvT when it becomes liberator coutner.
For comparison, Vikings do 19.6 single target DPS vs armored air, while thor offers 25 single target dps. Thors cost 6 supply while vikings cost 2 and is more mobile. There is no real incentive to make thors for the anti air capacity when it does so poorly. It had its niche as AA vs muta in past, now it doesn't do anything much.
---
build diversity:
Also, opening factory is difficult now with 2 base muta becoming a much larger threat- turret+thor isn't enough anymore vs 2 base muta openers since thors do so poorly against mutas as it became a single target 17.5 dps unit-
What made thors great vs muta was the splash and also the burst damage of thor that punished mutas flying into base. Thor offered an instant, splash damage response but with change, it doesn't anymore. Thor right now offers 14 dps over 3 seconds with small splash, while the change makes thor a 17.5 dps SINGLE target over 2 seconds, which is whole lot worse against mutas.
Not only is it less burst and instantaneously punishing. it loses the splash damage for very minor 3.5 dps upgrade.
Its worse even vs magic boxed muta as it loses the burst damage. Factory openings are simply discouraged with this change going through.
Also, however low the damage thors did against non-light units, it still offered reliable splash vs air units lategame. The single target AA numbers right now really don't offer that much in comparison to what you are losing out. It will no longer be able to punish clumped up units such as ravens/muta/viking ball.
---
lategame
Here is where thor change will shine at its best- as it is losing early-mid game utility/niche vs mutas.
However, as I did state with my first post, there is no real reason to use thors over vikings other than v.s. corruptors- where even then, it will not offer game changing damage when compared to damage output of AA marines can offer or even vikings.
The clunkiness of thors with its delayed shots and need to be constantly moving to retarget will hurt its dps during battles, as well as the size of thors not being able to focus on a single target without taking huge amount of space on battlefield.
Vs Broodlords- thor has 10 range while broodlords have ability to kite with broodlings with its 10.5 range and the slight "leash" range that allows broodlords to hit from longer than 10.5 range. This will prove difficult for thors to deal with since thors need to be constantly hitting broodlord every 2.1 seconds to get the damage out when there are things like broodlings blocking the pathing.
As some people will say, this change will help thor become "better" vs armored air units than it is now, which is slightly debatable as it is losing splash damage, but it would still require heavy support in form of bio/vikings to deal with opposition air units- but considering this whole change is done to make viable factory AA unit, it doesn't really seem to be doing what it is supposed to be doing.
If you tested it v.s. skytoss, it really doesn't trade that well at all in equal cost- for a specialized gas-intensive single target anti air factory unit, it trades unfavorably-especially vs tempests and carriers.
TL;DR:
In my opinoin, if anything, if Blizzard intends to go ahead with this change absolutely, Blizzard really needs to tweak the numbers at least to make up for the loss of Thor niche it had previously.