Hmmmm. I see, I see....Hmmmm.

I've never really played around with the demo disk before, but chances are if you're playing loops - there's no way to change their speed without getting into TIME STRETCH. This isn't a 'beat box' remember.
You can fool with 'speeds' using VARI-PITCH, but there's no way to internal save your changes on the 808 (No RAM). You'll have to record the pitch changes to a MD, DAT and resample back into the 808 that way.
But if the beat thats sampled says 160BPM - it stays 160, regardless of changing SONG tempo to 142, etc.
Changing tempo will only affect the duration of the sample length.

Think of it as dealing with little pieces of reel to reel tape. TIME STRETCH is a feature not found on many tape recorders, so that's a bonus for the 808. BUT, you can't deviate too much from the 'recorded BPM' or you'll make the loop sound phasey and choppy. Sometimes this can be put to great use as an effect.
So - if you want to change a 160BPM loop to 141BPM using TIME STRETCH, chances are it may sound hideous.
Try to find a good loop more in the range you're after. Usually 135 to 145BPM can flex well to 141BPM as an example.

And as you stand now, version 1.0 isn't going to hurt you and your Zips any, and is probably a good way to get familiar with your 808 to start. Later versions have more MIDI and FX upgrades that are great for sure - but maybe adding extra confussion if you're just starting out. Hopefully this explains things better for you? Good luck!