It should be OK, but it has never been proven in court for an RPG. What we call the "mechanics" of the spell can't be copyrighted, just the expression, i.e. fluff of those spells. This depends on an extrapolation of the idea the game mechanics are not copyrightable as establish law for board games and the like. There game mechanics mean something akin to moving a taken or rolling dice. It is not clear what courts will determine are game mechanics when it comes to TTRPGs. I think it is safe to assume it may not be what we generally call them.I guess this is the part I am unfamiliar with when it comes to copyright. While there are some spells in LU that have different effects to 5e, there are many spells where the changes are aesthetic only. Sure a few words are changed here and there, but the actual mechanical creation is exactly the same as what's in the SRD. Is that not problematic?