I am trying to translate the following phrase into Latin: "Be ready to change your goals but never change your values". I've got up to "Esto parata mutare propositos tuos sed numquam mutare valores tuos" but I'm not sure whether to use ablative instead of accusative after "mutare". Subject of the phrase is feminine singular.
It's good to see English to Latin coming back into favour - it really makes you think, though we don't see that much of it today.
I'd say the 'esto' is probably not needed - a bit like saying 'ego amo' instead of just 'amo'. But if the phrase is to be a motto, then yes.
On the main points, paro is 1st conjugation, and Kennedy 120 tells us the form for the imperative (to one person) is 'parator'.
muto is an ordinary transitive verb and thus demands the accusative. Maybe it could mean 'make changes to', in which case the dative would work. What does your grammar (Kennedy?, Allen & Greenough?) say?
An imperative like this doesn't really have a 'subject', masculine or feminine, so it would look the same if addressed to a boy or a girl, but different for a group.
Finally (!), the negative injunction 'do not change' might be better as 'ne mutes'
"Parare" takes the accusative and infinitive just like you've done.
You can use "parata esto", as you've done, or "parator" as John White suggested. The nuance between these two choices might be too small to worry about in a lapidary motto.
There are other options, of course. As an example, I'll throw out, "Parandum est mutari semper proposita, nunquam valores". Here I've replaced the imperative with a neuter impersonal passive gerundive, and changed "propositos" to neuter.