Hyster (not Hister as far as I can tell) is a registered trademark in the US. Regardless:

"The HYSTER trademark is filed in the category of Education and Entertainment Services" [0], so you can safely ignore any demands to rename, as it doesn't conflict. Embarrassing for them that their lawyers don't understand even the basics of trademark law.

Edit to clarify as some folks here are as confused as sHyster's lawyers: Trademarks are not exclusive, they're restricted to a category or categories. You can be Apple in the category of computers, but not in music if there's already an Apple in that category (unless you have $500M to spare [2]).

  [0] https://www.trademarkia.com/hyster-77843354
  [1] https://tmsearch.uspto.gov/ for a more authoritative source than [0], but it doesn't allow deep linking
  [2] https://en.wikipedia.org/wiki/Apple_Corps_v_Apple_Computer