I don't see any reasons for changing the names. All it does is create confusion and/or ruin themes, like "Alphamon and Omegamon" (beginning and end) to "Alphamon and Omnimon" (beginning and all) or "Lilithmon" (mother of demons and demon of lust) to "Laylamon" (I don't think this actually has a meaning other than being a typical prostitute's name). It's clear that BoA doesn't really want to change them either since they were told to change the names of the toys for "Daemon" and "Cherubimon" to "Creepymon" and "Kerpymon" respectively, and then changed them back to normal in the shows. I don't see any reason to change these names because in the actual Japanese (even the written names on toys and books) they just use English words spelled with the Japanese Hiragana (their alphabet).
With the terminology, I somewhat understand why they changed the names of the levels to focus less on life and more about fighting (since that's what the kids liked) but I always thought the main theme was something to do with bonding and growing up or something. And what was the point of making the name of the penultimate level the same as the Japanese name for the final level? Furthermore, what was their obsession with the prefix "digi-"? It seems like they just wanted to stick it in front of everything and all it did was become obnoxious and cliché.
Anyways, enough ranting, I want to hear why you think was the reason for renaming everything. I think for certain names ("Sleipmon" to "Kentaurosmon" or "Duftmon" to "Leopardmon") they just wanted to make them easier to remember since most kids don't understand what "sleip" or "duft" mean, and for some ("Chimeramon" to "Kimeramon" or "Diablomon" to "Diaboromon") they just wanted names that sounded catchier. For some other examples though ("Piyomon" to "Biyomon" or all the stuff from the first paragraph), I have no clue.
For the human names I understand they just wanted to make sure kids would be able to pronounce or remember them.