I don't know about Monster stat changes in Ragnarok Travels since I haven't got time to play, but I did notice that the Prontera church map was recently reverted back to the original style instead of the incorrect episode style it had in the beginning. I for one would welcome corrections like these any time, even if they are late.
There is nothing wrong with that in my opinion. If you're of the opinion that there's something wrong with "better late than never", then that's fair, but mistakes happen. I've seen it happen in many other RO servers where they had to correct something after players had already "taken advantage" of it.
Even if it was something radical like 999x more exp from the beginning, then a month later corrected down to 1x exp, it's still better late than never. At least I believe it is.
Should new players feel entitled to be on the same level as those who began a month earlier than them anyway? No, and Ragnarok Online shouldn't be a rush to the end of the game anyway. It's not supposed to be a race, and once people reach max level, it's not supposed to be over and done with. It's supposed to be a social game of cooperation to take down giant monsters or conquer WoE castles for your guild which you can always get to eventually, regardless of when you joined.
With any half-decent community, the ones at higher levels would want to help out new players most of the time anyway. Giving them help leveling or in WoE. Maybe even giving them items.
Even players who all joined at the beginning can have vastly different amounts of spare time to play, giving some players more advantage than others to progress faster. Some players are level 50 by the time others are still level 20 for example. Nothing wrong with that, and it shouldn't deter anyone from playing.