Welcome aboard. I agree with Bernie, 55 pounds is pretty heavy when making the switch. It can lead to lots of form issues. The bow I shoot most often is my 46# Kanati, and it will kill anything in Iowa or just about anywhere else in these United States.
You don't mention just what "longbow" you are looking at, but there is a vast difference within that category as well. My Kanati is technically a hybrid. It's a 58" forward handle reflex/deflex bow with trapped limbs and a grip similar to what you are used to on your compound (I speak bow, so if you need a translation do a search here). Since the string doesn't contact the limbs when it's braced, it is still more or less a longbow. My Mahaska is a mild r/d 62" bow that braces with the traditional D shaped profile, and it has a locator grip (much straighter than you are used to). It is much more like a traditional longbow, but since the limbs are flat in cross section, I suppose it is technically a flatbow, though flatbows do have wider limbs than this bow. I have a Northern Mist Shelton coming soon. It is a 66" Hill style longbow with string follow limbs and a straight grip (much different than you are accustomed to). It to will have the D profile, but it is not a traditional D shaped longbow since the cross section of the limb is flat on the belly and back (belly=toward the shooter/back=away). The "true" longbow is the English style which has a D shaped profile and a D shaped cross section and which flexes through the entire length (think Robin Hood here). That is a brief attempt at explaining the differences in longbows.
Each of these "longbows" are shot somewhat differently and may be shot radically differently than a compound as Pavan pointed out. Learn as much as you can before you buy. I am surprised that a traditional shop would not advise you to start with a much lighter weight than 55 pounds. Remember, with your compound at full draw you will be holding MUCH less weight than that. It can lead to poor form, frustration, and worst of all a return to wheels!
Anyway, glad to have you here. Have fun! This is the greatest forum on the net, bar none.