You definitely can relabel legally. There are certain things that you would have to include in your custom label of coarse. In order to have your own custom label you have to include (Fiber Content ex. 100% Content, Country of origin ex. Made in USA, RN number usually 5 digits, and instructions on how to wash, ex. Wash cold tumble dry etc. etc.) The rn number identifies which company manufactured the clothing ex. Hanes 15763. Hope that helps out a bit. Everyone definitely added some great insight in regards to what you can do as an alternate but if your going to run you own clothing line or something like that there is nothing my professional than a custom printed label.
If you have any questions feel free to pm me.