TLDR: Personal pronouns (I, you), interrogative pronouns (who, what), quantifiers (all, many, one) , negative particles (not) and determiners (this, that) are less likely to change over time than most nouns, adjectives and verbs.
There is a broad distinction between open-class and closed-class words. Open-class are those that easily admit new members, such as verbs, nouns and adjectives. They also consist of a large number of individual words - It's a daunting task to count all the English nouns listed in any given dictionary. Closed-class are those that do not readily admit new members, such as personal pronouns (I, you), interrogative pronouns (who, what), quantifiers (all, many, one) , negative particles (not) and determiners (this, that). They also consist of a rather small number of individual words - It's easy to count all the personal pronouns of the English language.
Most individual closed-class words also occur much more frequently in oral and written language than most individual open-class words. Now, frequency of occurrence happens to be one of the main factors influencing likelihood of historical change in linguistics (see, for example, Diessel (2007). Basically, the mental representation of a word is reinforced every time you hear or use it, and children are less likely to come up with innovations in language acquisition, if they hear a certain word very often. It is not true, as @Darkgamma stated, that
Nothing is inherently more conservative than anything else because pretty much every word is changed as time goes on.
Yes, every word is changed as time goes on (which is a big problem for linguists trying to learn anything about the prehistory of human languages), but they don't all change at the same rate. Historical linguists trying to determine how various languages are related exploit this by comparing words that are least susceptible to change - see for example the famous Swadesh list (although perceived cultural universality was an important consideration in choosing this list). Note that this list contains many personal and interrogative pronouns, negative particles and determiners. It also contains many verbs and nouns, but again these are frequent words referring to basic concepts.
So personal and interrogative pronouns, quantifiers, negative particles and determiners - as a group - are less likely to change over time than nouns, adjectives, and verbs - again, as a group. But this is not due to any inherent quality of pronouns and the like, but because of frequency of occurrence.
