Well, I don't know the technical details about the engine, but I will share my thoughts based on ideas of cost efficiency.
If something is NOT broken, don't fix it.
If the current engine used by you is sufficient to handle the game content you are going to ship with your future games (dynamic world, quests, progressive world development, etc.), just keep with it.
TBH, when we get into business aspect(s) of game development, it's all about budget management.
From OP, it seems the main motive of engine change is just for graphic enhancement.
The fancier the graphic, the higher the development cost, and hence less time available for game design, and gameplay usually suffers a lot in the end. This is what the "AAA" developers go for these days, as they only strive for "1st week/month sales" via promotion with "impressive graphics / videos".
But your fans base is not attracted by "fancy graphic" when you first publish your games, so it's better off just to spend your time further enhancing the game play experience instead.
For an indie developer, long-term support from a loyal fan base is much more crucial.