Why does the verb "succeed" change to "successor" as a noun?

Solution 1:

It comes from Latin successor, which means roughly the same. The verb comes from succed-o, which also means roughly the same.

The root of the Latin verbal stem ced- ("move away, cede") is the Proto-Indo-European root *ke-, "here, this" (deictic/pronominal stem), plus *s(e)d-, "sit, set, settle" (compound *k̂e-zd-ō)—so says the Oxford Latin Dictionary (1982): Pokorny and Ernout (1959) are unclear.

The prefix sub-, "from under", is added to form the meaning "to go sit/move from under" for succedo, if you will forgive me this awkward translation. It is a phonological rule in Latin that -bc- is normally assimilated into -cc-.

The supine stem of a Latin verb—the stem used to make past participles—is normally formed by adding -t- to the verbal stem: *cedt-. Phonology demands that -dt- (and -tt-) at the end of supine stems normally change into -ss-, hence the supine stem success-. The past participle is successus, roughly "succeeded".

The suffix -or is used in Latin to make an agent out of a verbal stem, just as in English -or and -er: hence Latin successor, "successor".

In English, we sometimes borrow words from Latin directly, or from Latin through French, like successor, while at other times we only borrow the verb and make our own agent word, like revoker (instead of revocator). But we usually use the Latin noun if you also use the Latin verb: proceed, processor; permit, permission, etc. etc.; sometimes we use both, as in merge, merger, immersion.