1) If you have a written lease, she cannot change its terms--i.e. change it to a "no pets" lease--until the end of the current lease; at that time, the next (or renewal) lease can have different terms, like no pets, unless and only to the extent that the current lease provides that it will renew on the same terms under certain circumstances, and those circumstances occur (e.g. it will renew under the same terms if you give notice 60 days prior to the end that you are renewing).2) If you have no written lease, you are on an oral month-to-month lease, and the landlord may change the terms on 30 days notice--even to make it no pets after you've had pets for 6 months.3) Notwithstanding the above, if the ferrets are in fact provably annoying other tenants (e.g. there have been complaints) or making the unit unhygienic (e.g. creating a real oder issue, which can be proven, if necessary, by credible testimony), then she can require you to remove them or otherwise abate the problem, because one tenant may not disturb other tenants. There are some limitations to this: first, the issue must be real and provable; and second, the complaint must be reasonable--that is, whatever the condition or issue is, it must be such as to disturb the average reasonable fellow tenant.