The main point is if the landlord or tenants will be Jewish and maintain the mezuzahs. In any case, you may change the (expensive) cases for cheap cases as the case is not the mezuzah but a way of keeping it safe or an adornment. There are many sources on the net and here is one of them.
Mezuzahs While Moving
CHAPTER 7
Mezuzahs While Moving
QUESTION: What if I am in the midst of moving and I temporarily own
two homes. Which one requires mezuzahs?
ANSWER: If you are actually living in both homes, or storing items in
both homes, then you are obligated to have mezuzahs in both homes.
QUESTION: Am I obligated to leave my mezuzahs up after moving?
ANSWER:
When moving to another house or apartment, mezuzahs should not be
removed, unless:
The new tenant is a non-Jew, A new tenant did not move in, and there
is concern that were the mezuzahs to be left behind, in all
probability they would become defaced.
QUESTION: May I remove my
mezuzahs if the new Jewish tenant has his own mezuzahs?
ANSWER: It is preferred that the new tenant or owner should remove
your mezuzahs, or he should at least instruct you to remove your
mezuzahs. This halacha is a very serious matter and should not be
treated lightly. Rather, an observant Rabbi should instruct you what
to do.
QUESTION: What if the new owner is Jewish and he wants my mezuzahs, am
I obligated to leave all of my mezuzahs behind?
ANSWER: Yes. You may, however exchange the mezuzahs themselves for
less expensive ones, providing that they are 100% kosher. The mezuzah
cases may certainly be exchanged for the least expensive mezuzah
covers.
You may also demand the new owner or tenant to pay you for the
mezuzahs. He will then be obligated to compensate for the mezuzahs at
fair market value. If he refuses to pay for them, an observant Rabbi
should be contacted. This Halacha is a very serious matter.