10HD, 30HD, and 50HD are based on the SAE grade's 10, 30, and 50 respectively which shouldn't have changed.
The HD relates to a detergent additive which might have changed however. The safe option is probably to go with straight oil.
If you're not sure what was in there beforehand, keep in mind that there were concerns in the move to HD oils as to the amount of crud that would be churned up and block oil lines and filters. If it's run on straight oil for a while you may have to observe similar precautions if moving to a detergent oil.