I don't really have any good sources. I know that a certain amount of lexical replacement is normal as languages evolve. People start saying "frater germanus" all the time, and then they drop the "frater" part, at which point they're just using "germanus" (or whatever its early Romance reflex was) to mean "brother". But changes like this tend to be pretty arbitrary. Spanish kept the Latin word for "cheese" (cassius, which became queso), while French and Italian replaced it with fromage/formaggio. All three replaced the Latin word for "horse", equus, with a different stem, caballo/cheval/cavallo. There doesn't appear to be much rhyme or reason to it.