When you go through the process of brining Vanya all the items the UI pops up with your active pet and a big blue button that says Change Pet.

I had inteprited this as I was changing which pet I wanted to transform and hastily changed my active Experiment 625 into a Nostalgic pet by mistake.

Maybe if it said Transform Pet instead of Change Pet it might prevent a similar mishap in the future.

(I already miracle Potioned back to a E625)

I can definitely see where somebody can interpret it that way. It's been a while since I got my nostaglic pet... does the process ever specify that it affects the active pet that gets changed?

i just made a pet Nostalgic the other day, and i believe it does, but in regular text where it may be easy to skim over or miss -- to the extent that i don't quite remember if i'm right or not. i'd already set him as my active just in case it did work like that. 😅

that being said, i'm sorry to hear about that and happy you got him back OP! i've been through a similar mishap (accidentally changed an Angelic pet into Galactic and had to potion them back; felt really silly about it for a while) and i'm definitely for some UI changes regarding pet colors obtained in a way that doesn't involve using a dropdown menu etc. different verbiage like 'Transform Pet' or even 'Change This Pet' would absolutely be handy on its own, but i think a useful additional update may be making anything that targets your active pet or requires a quest (which you may come back to, in the case of Galactic especially, forgetting which pet was chosen for it if it takes a while) state on the page in bold/large/colored font which specific pet it's going to change. i'm not sure what sort of coding would need to go into that, but even just applying some more emphatic font styling on any preexisting text that mentions your active pet would probably be helpful!

The site features that change your pet are inconsistent. For Nostalgic pets, it's your active pet, but for the Rift, if I remember right, I think you get to choose which pet to send in. I think you also get to choose which pet to turn Galactic, but strangely this is decided before you turn in the crystals, which makes it easy to forget if you don't have them all and need to do some shop searching. The Aenoid seed also changes your active pet, and unlike Vanya's, has no warning or even an "Are you sure?" pop-up. I've seen many users who've lost their hard-earned pets to the Aenoid seed.

What I'm saying is, I think these should all just be the same thing, preferably getting to choose your pet rather than defaulting to your active pet (I don't know about the Zapper, though), or to at least have a warning if it is going to change your active pet. Even just showing a picture of the pet that's going to be changed would work (don't know if that can be done for the Aenoid seed, though).

i agree that all pet transforming features should just let you pick the pet you want to change.

the active pet feature is a bit strange, and different people have different reasons for which pet will be their active one at a given time. making you switch it just to do one thing and then switch it back to do something else feels like a relic of older pet sites, like that's the way it works because That's Just How It's Done On Petsites, right?