You and your players will need very little effort to learn the mechanics of HARP. It is, at the same time, more detailed and more easy than MERP. You'll see that after a few game sessions.
The most important change is probably the One-Table-For-Everything (well, almost. Combat uses his own tables) and the Attacks-With-Just-One-Roll.
As for compatibility with older MERP products, the stats are impossible to convert. You can use the setting, the adventures, the NPCs, etc. like with any other product, but not the statistics. Only the modifiers related to difficulties of Maneuvers (Perception rolls, Disarm Traps, etc.) can be used without changes.
For a Middle-Earth adaptation for HARP, I made some documents myself I use in my adventures. If you have any interest in them, I can sent you a PM with them.