Again I'm pretty much brain-dead on Echos or 808s other than what I've read on the forum, but seems to me if they both have the 808 (I'm thinking this would be KR808D-1) threading and the resistance is compatible the cartos would work interchangeably. I do know you do *not* want to use a low resistance carto on a slim (or low mAh) battery, for the sake of the battery. I'm just guessing here but it's possible an extension type adapter might be needed to use standard KR808 cartos on your Echo... hopefully Swedishfish will be back with more info on this. She's one of the very knowledgeable folks on here about 808 stuff!
I would suggest, if possible keep your slim batt stuff up & running for "spare" service if nothing else. But you probably already knew that I guess.
Happy vaping! 
I would suggest, if possible keep your slim batt stuff up & running for "spare" service if nothing else. But you probably already knew that I guess.
