Dinah:
A pretty good description of Dinah would be to take Lucas the spider, scale him up to the size of a 12 YO girl, and attach Dinah's upper torso to him. Her skin has paled a bit, like she's spent too much time inside, her hair has been lighted to a sand color, and her brown eyes have turned a purplish-red color. Her ears have a slight point to them as well. No, she's not getting any spell casting abilities. Yes, she keeps her precognitive abilities. Yes, she has all the normal abilities of a drider (Makes silk & webs, can climb walls and ceilings, is not slowed down/trapped by webs, is poisonous, and so on). Her version of Driderism is not the curse version; Dinah retains her gender, isn't a bloated monstrosity with a death wish and a hate on for Drow and so on. The one thing her is that Dinah will have a very long life; how long I've yet to determine - either the life span of a half elf, or that of a drow. Her aging will slow down after puberty.
Her transformation was painful because Lolth is all about pain and strife and is still an evil bitch; if she's doing something kinda-good, it's because she's planning something in the future that's going to be nasty. Dinah will suffer, though it will be having to deal with how people treat her rather than being addicted to drugs and forced to answer Coil's questions.
Lisa:
A good description of Lisa would simply be Lisa, with a pair of fox ears and five fox tails. Her human ears are replaced by the fox ears. As a fox, she's a blonde fox (I've seen foxes in black, grey, Red, sandy brown, and 'blonde' (a golden brown). We've got black coyotes as well). Yes, she keeps her powers, which glitches out on her new form, liking the fluffy. Yes, she can go from full human, to human with fox ears, to full on anthro fox (do not google this, you will be very sorry, and Brain Bleach is currently not available because people have bought up all available supplies), to full fox. She has to spend time fuzzy every so often. If she gains any levels, it will be as a rogue. The one power she does gain is flying (though really, it's more like floating). She's able to jump and climb like your average anime ninja, and can hide really well.
Why:
Lisa and Dinah are two of the more powerful Thinkers in the setting. The Antagonist (who a number of people have tagged as Falazure) is looking to gather resources to further his schemes, and a precognitive and someone who can piece together things from a few scraps of data are incredibly useful assets for a villain to have. Lolth and Tamamo-no-Mae are two powers who do not wish to see that happen. So, by claiming Dinah and Lisa's souls, they keep them out of the Big Bad's reach for the moment, since right now, both are more powerful than him (Tamamo moreso than Lolth; Tamamo is back by Inari and Amaterasu (who may or may not be running around as a white wolf) while Lolth is backed by no one).
Lolth's claim on Dinah is more tenuous than Tamamo's claim on Lisa; Lisa was given a choice and took it of her own free will. Lolth forced it on Dinah. This may be my Judeo - Christian upbringing showing, but anything regarding your soul has to be agreed to by the person whose soul it is. And therein lies Dinah's out, and Lolth knows it. Should Dinah discover it, and take it, she can choose to go back to who she was, or stay the way she is, and not have to serve Lolth at all. If Dinah chooses to serve Lolth anyway, that seals the deal. Regardless of which way she goes, Lolth gets to watch a little girl suffer, which is cruel, but not the worst thing she's done, and maybe ask a few questions now and again.
Of course, this will annoy the Big Bad, and delay his schemes.