Has anyone ever seen an oracle ‘Change’ a monster into an XL (boss type) monster?
Didn’t even seen any oracle use that sklll in the first place. 
Ummm did you maybe run into a field/world boss?
I don’t think an oracle can change the type like that. I think they can make things really high hp but I doubt they can make bosses appear from normal enemies.
On iToS is not possible, i changed so many monsters and when i get something strong is like @Deathvale said, a normal monster with high HP pool.
On KR version i can’t be sure, they are far ahead than us in patches, you never know when they changed that in there.
Also, some bosses spawn small version of themselves, Sequoia, Salamander and Werewolf do that, they are considered normal monster but for Transform skill, it may be codded to work as a boss monster.
it is impossible to get boss monster through change because they are immune to change. However it is possible to get certain quest ‘boss’ such as red vubbe fighter though change. They are big but really weak…
I think there’s a possibility if that XL type monster is one ET/Solmiki monster and isn’t disabled on oracle change skill somehow.