It took me a little while to understand what you are asking here; it appears that your question is based on the Abbott translation, which is almost a century old now.
The passage in question does not equate "conservation" with "happiness". Let us look more closely:
Now in a being which has reason and a will, if the proper object of nature were its conservation, its welfare, in a word, its happiness, then nature would have hit upon a very bad arrangement in selecting the reason of the creature to carry out this purpose.
What is spoken of here is not conservation in the abstract, but of a being's conservation of itself, i.e., self-preservation. Furthermore, happiness is not being equated with self-preservation; rather, it is the notion of self-preservation plus the welfare of the being. Finally, all of this is appearing within a counterfactual.
This is all made more clear by looking at the same passage in a more recent (and more informal) translation, by Bennett:
Now suppose that nature’s real purpose
for you, a being with reason and will, were that you should
survive, thrive, and be happy—in that case nature would
have hit upon a very poor arrangement in appointing your
reason to carry out this purpose!
I suggest that you consult some other translations if you run into future difficulties.