Any weapon you "upgrade" to overwrites the last. So if you go from a weapon with lesser stats to more, the effect will carry over. However if, for example, your current B-Rank weapon has 10 points into Attack [and it being fully upgraded] and you upgrade to an A-Rank one with only 8 points into Attack, you'll lose those two points. You can always upgrade to any weapon you want [assuming they can use that type], but know that the previous one will be lost.So if I got 3* sword, I can evolve it again? Like, there is no limit right?
Traditionally, you save the premium B-Rank weapons for those that will last you a while, so only 4*/5* units. If you get an A-Rank weapon, definitely be careful on who you use it on since there's no way to "remove" a weapon after you fuse it.
In the current Japanese version, you can equip multiple weapons to the same character and switch between them, but that's not available yet in the English version.