This is a good point. Conversions shouldn't penalize characters; if a Rogue could previously qualify at, say, 5th level, then he should still be able to qualify at the same level after conversion.
Adding Diplomacy as a pre-req isn't necessary, though. The Royal Explorer required 5 ranks in Decipher Script, 8 in Intuit Direction, 8 in Profession cartographer, and 5 in Wilderness Lore (plus Alertness and Endurance).
Looking at the class, I think it's weak. A rogue gives up all sneak attack advancement, for 10 levels, which IMHO is a terrible choice. A ranger gives up good BAB. A bard gives up all spell advancement. The class offers nothing that's fair compensation for these losses. These are the 3 classes most likely to become Royal Explorers.
Thus, I'd be inclined to be kind in converting the requirements: 5 ranks in Decipher Script, 8 in Profession, 5 in Survival, and the 2 feats.