I highly doubt there is a problem or that Epic care.If you are paying than its yours to use.And if you mean the free assets, again, I doubt there will be a problem because Epic says Unreal engine use and we are using unreal engine and we are so few and they are so flexible.At the end of the day, we pay the same royalty to Epic as ue4 users.In fact, the license I bought for udk is the same as of ue4. in terms for royalties.
As I said, i dont think they will care.We purchased it, used it with a unreal engine version and payed them the same royalties as the newer versions of unreal users.Everything seems perfectly legit to me.