They almost certainly won't disallow it. Almost no game has ever done this. Instead it means that if you are below the "recommended" requirements and you have poor performance, then it is to be expected. The point is to not look at the 6 year old requirements and expect that an average computer from 6 years ago will handle Helms Deep smoothly.
Thank you Grimdi and Lohi. My main concern is that one of my systems was brand new the year the game was released. I have upgraded the RAM to the max (4 GB), and it runs LOTRO well right now, though not as smoothly as my newer system. If that system isn't going to run LOTRO in a month, maybe I should be more concerned with replacing it before purchasing the new expansion. Any suggestions on what might need to be upgraded to make HD playable or will I probably need to scrap the system?