Impermanence of self is a mystical realization, a knowledge you can both learn, and also embody. Realizing the Self, and merging with God, is another mystical realization that you can both learn and embody. One spiritual truth is not higher than another spiritual truth.
What characterizes spiritual truth is the second part, truth. It’s true whether you believe in it or not, whether you’re embodying it or not, whether you understand it or not. Truth cannot be higher or lower, it stays exactly where it is.
What changes is the being. You can move, but the truth will not. It matters not whether you are Buddhist or you are Christian. Embodiment of spiritual truth is not facilitated by movement.
Let’s pull back before we get too meta. The Bible speaks of impermanence of self. Ecclesiastes is one of the three books of wisdom canonized in the Torah. The lesson the book teaches is that everything that man does is hevel, or impermanent, vain, futile. Any castle man builds will be demolished at some point. Including the castle of self. One should simply take pleasure in the here and now.
This hammers the same point that Buddhism gets at. The truths do not change, only the methods for conveying them. Everything Buddhism teaches, everything any religion or school of thought teaches, is the same spiritual truth as what everything else teaches. You’re not going to find higher truth by changing your identity from Christian to Buddhist. You’re just going to find the same truth, worded differently.
I have no problem with broadening one’s field of study. But you, person who has realized the Self and merged with God, if you knew what those words really meant, you wouldn’t be throwing them around so casually, are not going to obtain anything by erasing the part of your D&D character sheet where you wrote your religion down, from one appellation to another one.
The growth comes from the depth of study, not in the silly effort to change truth.