Well, i don’t have experiences of my own but i can reply you based on what i read (and i read quite a lot because i was also curious about this question).
So - from what i’ve seen expiriences regarding look of succubi and incubi varies quite a lot from people to people. Even to the point when i would come out with a theory it really depends on your expectations.
Some people claim that succubi take form according to likings of their partner. Sometimes even grow attached to it and stick to it after some time.
Another people say that their succubi appear differently to them every time.
Others claim that succubi have their “natural” form yet they still can change their appearance.
In general it’s rather accepted fact that they are shapeshifters.
The common part more or less is that they can get “darker” apperance. Even in the case when they’re appear you as i mentioned in first scenario they still might do that. Some often, some rare - i guess it depends on their judgement if you’re ready, if you can handle it but if you trust one another i guess it will happen sooner or later. They’re dualistic in their being, that’s just how they are and there’s no need to be scared of it.
So - basicly in my humble opinion based on my research i would say it pretty much depends on what you expect. “As above - so below”
If you evoke goetic spirit - Astaroth for example with mindset that you are summoning “devil himself” - you might get that scary, devilish apperanace. On the othret hand you summon this spirit knowing that he was originally SHE - pagan goddes Astarte you might get that feminine, not demonic appearance.
I think its possible that it’s more or less simillar with succubi.
It can also depend on personality of succubus herself.